diff --git a/src/commands/build.ts b/src/commands/build.ts
index a4b34a6317..5319348d4c 100644
--- a/src/commands/build.ts
+++ b/src/commands/build.ts
@@ -228,6 +228,10 @@ export async function command(commandOptions: CommandOptions) {
   }
 
   const allBuiltFromFiles = new Set<string>();
+
+  const webModulesScript = config.scripts.find(script => script.id === 'mount:web_modules');
+  const webModulesLoc = webModulesScript ? webModulesScript.args.toUrl as string : '/web_modules';
+
   for (const workerConfig of relevantWorkers) {
     const {id, match, type} = workerConfig;
     if (type !== 'build' || match.length === 0) {
@@ -317,7 +321,7 @@ export async function command(commandOptions: CommandOptions) {
             if (dependencyImportMap.imports[spec]) {
               let resolvedImport = path.posix.join(
                 config.buildOptions.baseUrl,
-                `/web_modules`,
+                webModulesLoc,
                 dependencyImportMap.imports[spec],
               );
               const extName = path.extname(resolvedImport);
@@ -337,7 +341,7 @@ export async function command(commandOptions: CommandOptions) {
                 pkgName: missingPackageName,
               },
             });
-            return path.posix.join(config.buildOptions.baseUrl, `/web_modules`, `${spec}.js`);
+            return path.posix.join(config.buildOptions.baseUrl, webModulesLoc, `${spec}.js`);
           });
           code = wrapImportMeta({code, env: true, hmr: false, config});
         }
diff --git a/src/commands/dev.ts b/src/commands/dev.ts
index 6521081ad9..b9b7ba6ddd 100644
--- a/src/commands/dev.ts
+++ b/src/commands/dev.ts
@@ -287,6 +287,9 @@ export async function command(commandOptions: CommandOptions) {
         filesBeingBuilt.delete(fileLoc);
       }
     }
+    const webModulesScript = config.scripts.find(script => script.id === 'mount:web_modules');
+    const webModulesLoc = webModulesScript ? webModulesScript.args.toUrl : '/web_modules';
+
     const ext = path.extname(fileLoc).substr(1);
     if (ext === 'js' || srcFileExtensionMapping[ext] === 'js') {
       let missingWebModule: {spec: string; pkgName: string} | null = null;
@@ -319,7 +322,7 @@ export async function command(commandOptions: CommandOptions) {
         }
         if (dependencyImportMap.imports[spec]) {
           let resolvedImport = path.posix.resolve(
-            `/web_modules`,
+            webModulesLoc,
             dependencyImportMap.imports[spec],
           );
           const extName = path.extname(resolvedImport);
@@ -343,7 +346,7 @@ export async function command(commandOptions: CommandOptions) {
         if (extName && extName !== '.js') {
           spec = spec + '.proxy';
         }
-        return `/web_modules/${spec}.js`;
+        return path.posix.join(webModulesLoc, `${spec}.js`);
       });
 
       messageBus.emit('MISSING_WEB_MODULE', {
diff --git a/src/config.ts b/src/config.ts
index b586a837e4..43b007fb9f 100644
--- a/src/config.ts
+++ b/src/config.ts
@@ -316,6 +316,7 @@ function handleLegacyProxyScripts(config: any) {
 
 type RawScripts = Record<string, string>;
 function normalizeScripts(cwd: string, scripts: RawScripts): BuildScript[] {
+  const dependenciesLoc = process.env.NODE_ENV === 'production' ? BUILD_DEPENDENCIES_DIR : DEV_DEPENDENCIES_DIR;
   const processedScripts: BuildScript[] = [];
   if (Object.keys(scripts).filter((k) => k.startsWith('bundle:')).length > 1) {
     handleConfigError(`scripts can only contain 1 script of type "bundle:".`);
@@ -354,8 +355,15 @@ function normalizeScripts(cwd: string, scripts: RawScripts): BuildScript[] {
           `scripts[${scriptId}]: "--to ${to}" must be a URL path, and start with a "/"`,
         );
       }
-      const dirDisk = cmdArr[0];
+      let dirDisk = cmdArr[0];
       const dirUrl = to || `/${cmdArr[0]}`;
+
+      // mount:web_modules is a special case script where the fromDisk
+      // arg is harcoded to match the internal dependency dir
+      if (scriptId === 'mount:web_modules') {
+        dirDisk = dependenciesLoc;
+      }
+
       newScriptConfig.args = {
         fromDisk: path.posix.normalize(dirDisk + '/'),
         toUrl: path.posix.normalize(dirUrl + '/'),
@@ -379,15 +387,13 @@ function normalizeScripts(cwd: string, scripts: RawScripts): BuildScript[] {
   }
 
   if (!scripts['mount:web_modules']) {
-    const fromDisk =
-      process.env.NODE_ENV === 'production' ? BUILD_DEPENDENCIES_DIR : DEV_DEPENDENCIES_DIR;
     processedScripts.push({
       id: 'mount:web_modules',
       type: 'mount',
       match: ['web_modules'],
       cmd: `mount $WEB_MODULES --to /web_modules`,
       args: {
-        fromDisk,
+        fromDisk: dependenciesLoc,
         toUrl: '/web_modules',
       },
     });
diff --git a/test/build/custom-modules-dir/.gitignore b/test/build/custom-modules-dir/.gitignore
new file mode 100644
index 0000000000..378eac25d3
--- /dev/null
+++ b/test/build/custom-modules-dir/.gitignore
@@ -0,0 +1 @@
+build
diff --git a/test/build/custom-modules-dir/index.test.js b/test/build/custom-modules-dir/index.test.js
new file mode 100644
index 0000000000..67608ccf7b
--- /dev/null
+++ b/test/build/custom-modules-dir/index.test.js
@@ -0,0 +1,19 @@
+const fs = require('fs');
+const path = require('path');
+const execa = require('execa');
+
+it('custom web_modules folder', () => {
+  execa.sync('npm', ['run', 'TEST'], {
+    cwd: __dirname,
+    // override NODE_ENV=test from jest, otherwise snowpack will assume
+    // development mode and try to copy from DEV_DEPENDENCIES_DIR
+    env: { NODE_ENV: 'production' }
+  });
+
+  // should write modules to the custom folder on disk
+  expect(fs.existsSync(path.join(__dirname, 'build', 'my_modules', 'array-flatten.js'))).toBe(true);
+
+  // should rewrite path for web_modules imports
+  const outputJS = fs.readFileSync(path.resolve(__dirname, 'build', '_dist_', 'index.js'), 'utf8');
+  expect(outputJS).toContain('/my_modules/array-flatten.js');
+});
diff --git a/test/build/custom-modules-dir/node_modules/array-flatten/LICENSE b/test/build/custom-modules-dir/node_modules/array-flatten/LICENSE
new file mode 100644
index 0000000000..983fbe8aec
--- /dev/null
+++ b/test/build/custom-modules-dir/node_modules/array-flatten/LICENSE
@@ -0,0 +1,21 @@
+The MIT License (MIT)
+
+Copyright (c) 2014 Blake Embrey (hello@blakeembrey.com)
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.
diff --git a/test/build/custom-modules-dir/node_modules/array-flatten/README.md b/test/build/custom-modules-dir/node_modules/array-flatten/README.md
new file mode 100644
index 0000000000..d35e67bdf8
--- /dev/null
+++ b/test/build/custom-modules-dir/node_modules/array-flatten/README.md
@@ -0,0 +1,43 @@
+# Array Flatten
+
+[![NPM version][npm-image]][npm-url]
+[![NPM downloads][downloads-image]][downloads-url]
+[![Build status][travis-image]][travis-url]
+[![Test coverage][coveralls-image]][coveralls-url]
+[![Bundle size][bundlephobia-image]][bundlephobia-url]
+
+> Flatten nested arrays.
+
+## Installation
+
+```
+npm install array-flatten --save
+```
+
+## Usage
+
+```js
+import { flatten } from "array-flatten";
+
+flatten([1, [2, [3, [4, [5], 6], 7], 8], 9]);
+//=> [1, 2, 3, 4, 5, 6, 7, 8, 9]
+
+(function() {
+  flatten(arguments); //=> [1, 2, 3]
+})(1, [2, 3]);
+```
+
+## License
+
+MIT
+
+[npm-image]: https://img.shields.io/npm/v/array-flatten.svg?style=flat
+[npm-url]: https://npmjs.org/package/array-flatten
+[downloads-image]: https://img.shields.io/npm/dm/array-flatten.svg?style=flat
+[downloads-url]: https://npmjs.org/package/array-flatten
+[travis-image]: https://img.shields.io/travis/blakeembrey/array-flatten.svg?style=flat
+[travis-url]: https://travis-ci.org/blakeembrey/array-flatten
+[coveralls-image]: https://img.shields.io/coveralls/blakeembrey/array-flatten.svg?style=flat
+[coveralls-url]: https://coveralls.io/r/blakeembrey/array-flatten?branch=master
+[bundlephobia-image]: https://img.shields.io/bundlephobia/minzip/array-flatten.svg
+[bundlephobia-url]: https://bundlephobia.com/result?p=array-flatten
diff --git a/test/build/custom-modules-dir/node_modules/array-flatten/dist.es2015/index.js b/test/build/custom-modules-dir/node_modules/array-flatten/dist.es2015/index.js
new file mode 100644
index 0000000000..a82b0460bc
--- /dev/null
+++ b/test/build/custom-modules-dir/node_modules/array-flatten/dist.es2015/index.js
@@ -0,0 +1,23 @@
+/**
+ * Flatten an array indefinitely.
+ */
+export function flatten(array) {
+    var result = [];
+    $flatten(array, result);
+    return result;
+}
+/**
+ * Internal flatten function recursively passes `result`.
+ */
+function $flatten(array, result) {
+    for (var i = 0; i < array.length; i++) {
+        var value = array[i];
+        if (Array.isArray(value)) {
+            $flatten(value, result);
+        }
+        else {
+            result.push(value);
+        }
+    }
+}
+//# sourceMappingURL=index.js.map
\ No newline at end of file
diff --git a/test/build/custom-modules-dir/node_modules/array-flatten/dist.es2015/index.js.map b/test/build/custom-modules-dir/node_modules/array-flatten/dist.es2015/index.js.map
new file mode 100644
index 0000000000..c49f36c2c5
--- /dev/null
+++ b/test/build/custom-modules-dir/node_modules/array-flatten/dist.es2015/index.js.map
@@ -0,0 +1 @@
+{"version":3,"file":"index.js","sourceRoot":"","sources":["../src/index.ts"],"names":[],"mappings":"AAcA;;GAEG;AACH,MAAM,UAAU,OAAO,CAA2B,KAAQ;IACxD,IAAM,MAAM,GAAiB,EAAE,CAAC;IAChC,QAAQ,CAAI,KAAK,EAAE,MAAM,CAAC,CAAC;IAC3B,OAAO,MAAM,CAAC;AAChB,CAAC;AAED;;GAEG;AACH,SAAS,QAAQ,CACf,KAAQ,EACR,MAAoB;IAEpB,KAAK,IAAI,CAAC,GAAG,CAAC,EAAE,CAAC,GAAG,KAAK,CAAC,MAAM,EAAE,CAAC,EAAE,EAAE;QACrC,IAAM,KAAK,GAAG,KAAK,CAAC,CAAC,CAAC,CAAC;QAEvB,IAAI,KAAK,CAAC,OAAO,CAAC,KAAK,CAAC,EAAE;YACxB,QAAQ,CAAC,KAAY,EAAE,MAAM,CAAC,CAAC;SAChC;aAAM;YACL,MAAM,CAAC,IAAI,CAAC,KAAK,CAAC,CAAC;SACpB;KACF;AACH,CAAC","sourcesContent":["/**\n * Pick the value from an array.\n */\nexport type PickValue<T> = T extends ReadonlyArray<any>\n  ? {\n      [K in Extract<keyof T, number>]: PickValue<T[K]>;\n    }[number]\n  : T;\n\n/**\n * Flatten an `ArrayLike` object in TypeScript.\n */\nexport type FlatArray<T extends ArrayLike<any>> = Array<PickValue<T[number]>>;\n\n/**\n * Flatten an array indefinitely.\n */\nexport function flatten<T extends ArrayLike<any>>(array: T): FlatArray<T> {\n  const result: FlatArray<T> = [];\n  $flatten<T>(array, result);\n  return result;\n}\n\n/**\n * Internal flatten function recursively passes `result`.\n */\nfunction $flatten<T extends ArrayLike<any>>(\n  array: T,\n  result: FlatArray<T>\n): void {\n  for (let i = 0; i < array.length; i++) {\n    const value = array[i];\n\n    if (Array.isArray(value)) {\n      $flatten(value as any, result);\n    } else {\n      result.push(value);\n    }\n  }\n}\n"]}
\ No newline at end of file
diff --git a/test/build/custom-modules-dir/node_modules/array-flatten/dist.es2015/index.spec.js b/test/build/custom-modules-dir/node_modules/array-flatten/dist.es2015/index.spec.js
new file mode 100644
index 0000000000..bc275d0367
--- /dev/null
+++ b/test/build/custom-modules-dir/node_modules/array-flatten/dist.es2015/index.spec.js
@@ -0,0 +1,42 @@
+import { expectType } from "ts-expect";
+import { flatten } from "./index";
+describe("flatten", function () {
+    it("should flatten an array", function () {
+        var result = flatten([1, [2, [3, [4, [5]]], 6, [[7], 8], 9], 10]);
+        expectType(result);
+        expect(result).toStrictEqual([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);
+    });
+    it("should work with array-like", function () {
+        var result = flatten("test");
+        expectType(result);
+        expect(result).toStrictEqual(["t", "e", "s", "t"]);
+    });
+    it("should work with readonly array", function () {
+        var input = [1, [2, [3, [4]]]];
+        var result = flatten(input);
+        expectType(result);
+        expect(result).toStrictEqual([1, 2, 3, 4]);
+    });
+    it("should work with arguments", function () {
+        var input = (function () {
+            return arguments;
+        })();
+        var result = flatten(input);
+        expectType(result);
+        expect(result).toStrictEqual([]);
+    });
+    it("should work with mixed types", function () {
+        var fn = function (x) { return x; };
+        var input = [1, ["test", [fn, [true]]]];
+        var result = flatten(input);
+        expectType(result);
+        expect(result).toStrictEqual([1, "test", fn, true]);
+    });
+    it("should work with tuples", function () {
+        var input = [1, [1, 2], [3]];
+        var result = flatten(input);
+        expectType(result);
+        expect(result).toStrictEqual([1, 1, 2, 3]);
+    });
+});
+//# sourceMappingURL=index.spec.js.map
\ No newline at end of file
diff --git a/test/build/custom-modules-dir/node_modules/array-flatten/dist.es2015/index.spec.js.map b/test/build/custom-modules-dir/node_modules/array-flatten/dist.es2015/index.spec.js.map
new file mode 100644
index 0000000000..3cf0329441
--- /dev/null
+++ b/test/build/custom-modules-dir/node_modules/array-flatten/dist.es2015/index.spec.js.map
@@ -0,0 +1 @@
+{"version":3,"file":"index.spec.js","sourceRoot":"","sources":["../src/index.spec.ts"],"names":[],"mappings":"AAAA,OAAO,EAAE,UAAU,EAAE,MAAM,WAAW,CAAC;AACvC,OAAO,EAAE,OAAO,EAAE,MAAM,SAAS,CAAC;AAElC,QAAQ,CAAC,SAAS,EAAE;IAClB,EAAE,CAAC,yBAAyB,EAAE;QAC5B,IAAM,MAAM,GAAG,OAAO,CAAC,CAAC,CAAC,EAAE,CAAC,CAAC,EAAE,CAAC,CAAC,EAAE,CAAC,CAAC,EAAE,CAAC,CAAC,CAAC,CAAC,CAAC,EAAE,CAAC,EAAE,CAAC,CAAC,CAAC,CAAC,EAAE,CAAC,CAAC,EAAE,CAAC,CAAC,EAAE,EAAE,CAAC,CAAC,CAAC;QAEpE,UAAU,CAAW,MAAM,CAAC,CAAC;QAE7B,MAAM,CAAC,MAAM,CAAC,CAAC,aAAa,CAAC,CAAC,CAAC,EAAE,CAAC,EAAE,CAAC,EAAE,CAAC,EAAE,CAAC,EAAE,CAAC,EAAE,CAAC,EAAE,CAAC,EAAE,CAAC,EAAE,EAAE,CAAC,CAAC,CAAC;IAChE,CAAC,CAAC,CAAC;IAEH,EAAE,CAAC,6BAA6B,EAAE;QAChC,IAAM,MAAM,GAAG,OAAO,CAAC,MAAM,CAAC,CAAC;QAE/B,UAAU,CAAW,MAAM,CAAC,CAAC;QAE7B,MAAM,CAAC,MAAM,CAAC,CAAC,aAAa,CAAC,CAAC,GAAG,EAAE,GAAG,EAAE,GAAG,EAAE,GAAG,CAAC,CAAC,CAAC;IACrD,CAAC,CAAC,CAAC;IAEH,EAAE,CAAC,iCAAiC,EAAE;QACpC,IAAM,KAAK,GAAG,CAAC,CAAC,EAAE,CAAC,CAAC,EAAE,CAAC,CAAC,EAAE,CAAC,CAAC,CAAC,CAAC,CAAC,CAAU,CAAC;QAC1C,IAAM,MAAM,GAAG,OAAO,CAAC,KAAK,CAAC,CAAC;QAE9B,UAAU,CAAoB,MAAM,CAAC,CAAC;QAEtC,MAAM,CAAC,MAAM,CAAC,CAAC,aAAa,CAAC,CAAC,CAAC,EAAE,CAAC,EAAE,CAAC,EAAE,CAAC,CAAC,CAAC,CAAC;IAC7C,CAAC,CAAC,CAAC;IAEH,EAAE,CAAC,4BAA4B,EAAE;QAC/B,IAAM,KAAK,GAAG,CAAC;YACb,OAAO,SAAS,CAAC;QACnB,CAAC,CAAC,EAAE,CAAC;QACL,IAAM,MAAM,GAAG,OAAO,CAAC,KAAK,CAAC,CAAC;QAE9B,UAAU,CAAQ,MAAM,CAAC,CAAC;QAE1B,MAAM,CAAC,MAAM,CAAC,CAAC,aAAa,CAAC,EAAE,CAAC,CAAC;IACnC,CAAC,CAAC,CAAC;IAEH,EAAE,CAAC,8BAA8B,EAAE;QACjC,IAAM,EAAE,GAAG,UAAC,CAAS,IAAK,OAAA,CAAC,EAAD,CAAC,CAAC;QAC5B,IAAM,KAAK,GAAG,CAAC,CAAC,EAAE,CAAC,MAAM,EAAE,CAAC,EAAE,EAAE,CAAC,IAAI,CAAC,CAAC,CAAC,CAAC,CAAC;QAC1C,IAAM,MAAM,GAAG,OAAO,CAAC,KAAK,CAAC,CAAC;QAE9B,UAAU,CAA4C,MAAM,CAAC,CAAC;QAE9D,MAAM,CAAC,MAAM,CAAC,CAAC,aAAa,CAAC,CAAC,CAAC,EAAE,MAAM,EAAE,EAAE,EAAE,IAAI,CAAC,CAAC,CAAC;IACtD,CAAC,CAAC,CAAC;IAEH,EAAE,CAAC,yBAAyB,EAAE;QAC5B,IAAM,KAAK,GAAyC,CAAC,CAAC,EAAE,CAAC,CAAC,EAAE,CAAC,CAAC,EAAE,CAAC,CAAC,CAAC,CAAC,CAAC;QACrE,IAAM,MAAM,GAAG,OAAO,CAAC,KAAK,CAAC,CAAC;QAE9B,UAAU,CAAW,MAAM,CAAC,CAAC;QAE7B,MAAM,CAAC,MAAM,CAAC,CAAC,aAAa,CAAC,CAAC,CAAC,EAAE,CAAC,EAAE,CAAC,EAAE,CAAC,CAAC,CAAC,CAAC;IAC7C,CAAC,CAAC,CAAC;AACL,CAAC,CAAC,CAAC","sourcesContent":["import { expectType } from \"ts-expect\";\nimport { flatten } from \"./index\";\n\ndescribe(\"flatten\", () => {\n  it(\"should flatten an array\", () => {\n    const result = flatten([1, [2, [3, [4, [5]]], 6, [[7], 8], 9], 10]);\n\n    expectType<number[]>(result);\n\n    expect(result).toStrictEqual([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);\n  });\n\n  it(\"should work with array-like\", () => {\n    const result = flatten(\"test\");\n\n    expectType<string[]>(result);\n\n    expect(result).toStrictEqual([\"t\", \"e\", \"s\", \"t\"]);\n  });\n\n  it(\"should work with readonly array\", () => {\n    const input = [1, [2, [3, [4]]]] as const;\n    const result = flatten(input);\n\n    expectType<(1 | 2 | 3 | 4)[]>(result);\n\n    expect(result).toStrictEqual([1, 2, 3, 4]);\n  });\n\n  it(\"should work with arguments\", () => {\n    const input = (function() {\n      return arguments;\n    })();\n    const result = flatten(input);\n\n    expectType<any[]>(result);\n\n    expect(result).toStrictEqual([]);\n  });\n\n  it(\"should work with mixed types\", () => {\n    const fn = (x: string) => x;\n    const input = [1, [\"test\", [fn, [true]]]];\n    const result = flatten(input);\n\n    expectType<(number | string | boolean | typeof fn)[]>(result);\n\n    expect(result).toStrictEqual([1, \"test\", fn, true]);\n  });\n\n  it(\"should work with tuples\", () => {\n    const input: [number, [number, number], [number]] = [1, [1, 2], [3]];\n    const result = flatten(input);\n\n    expectType<number[]>(result);\n\n    expect(result).toStrictEqual([1, 1, 2, 3]);\n  });\n});\n"]}
\ No newline at end of file
diff --git a/test/build/custom-modules-dir/node_modules/array-flatten/dist/index.d.ts b/test/build/custom-modules-dir/node_modules/array-flatten/dist/index.d.ts
new file mode 100644
index 0000000000..14dff30600
--- /dev/null
+++ b/test/build/custom-modules-dir/node_modules/array-flatten/dist/index.d.ts
@@ -0,0 +1,14 @@
+/**
+ * Pick the value from an array.
+ */
+export declare type PickValue<T> = T extends ReadonlyArray<any> ? {
+    [K in Extract<keyof T, number>]: PickValue<T[K]>;
+}[number] : T;
+/**
+ * Flatten an `ArrayLike` object in TypeScript.
+ */
+export declare type FlatArray<T extends ArrayLike<any>> = Array<PickValue<T[number]>>;
+/**
+ * Flatten an array indefinitely.
+ */
+export declare function flatten<T extends ArrayLike<any>>(array: T): FlatArray<T>;
diff --git a/test/build/custom-modules-dir/node_modules/array-flatten/dist/index.js b/test/build/custom-modules-dir/node_modules/array-flatten/dist/index.js
new file mode 100644
index 0000000000..16c4911af1
--- /dev/null
+++ b/test/build/custom-modules-dir/node_modules/array-flatten/dist/index.js
@@ -0,0 +1,26 @@
+"use strict";
+Object.defineProperty(exports, "__esModule", { value: true });
+/**
+ * Flatten an array indefinitely.
+ */
+function flatten(array) {
+    var result = [];
+    $flatten(array, result);
+    return result;
+}
+exports.flatten = flatten;
+/**
+ * Internal flatten function recursively passes `result`.
+ */
+function $flatten(array, result) {
+    for (var i = 0; i < array.length; i++) {
+        var value = array[i];
+        if (Array.isArray(value)) {
+            $flatten(value, result);
+        }
+        else {
+            result.push(value);
+        }
+    }
+}
+//# sourceMappingURL=index.js.map
\ No newline at end of file
diff --git a/test/build/custom-modules-dir/node_modules/array-flatten/dist/index.js.map b/test/build/custom-modules-dir/node_modules/array-flatten/dist/index.js.map
new file mode 100644
index 0000000000..3ab13a8dc1
--- /dev/null
+++ b/test/build/custom-modules-dir/node_modules/array-flatten/dist/index.js.map
@@ -0,0 +1 @@
+{"version":3,"file":"index.js","sourceRoot":"","sources":["../src/index.ts"],"names":[],"mappings":";;AAcA;;GAEG;AACH,SAAgB,OAAO,CAA2B,KAAQ;IACxD,IAAM,MAAM,GAAiB,EAAE,CAAC;IAChC,QAAQ,CAAI,KAAK,EAAE,MAAM,CAAC,CAAC;IAC3B,OAAO,MAAM,CAAC;AAChB,CAAC;AAJD,0BAIC;AAED;;GAEG;AACH,SAAS,QAAQ,CACf,KAAQ,EACR,MAAoB;IAEpB,KAAK,IAAI,CAAC,GAAG,CAAC,EAAE,CAAC,GAAG,KAAK,CAAC,MAAM,EAAE,CAAC,EAAE,EAAE;QACrC,IAAM,KAAK,GAAG,KAAK,CAAC,CAAC,CAAC,CAAC;QAEvB,IAAI,KAAK,CAAC,OAAO,CAAC,KAAK,CAAC,EAAE;YACxB,QAAQ,CAAC,KAAY,EAAE,MAAM,CAAC,CAAC;SAChC;aAAM;YACL,MAAM,CAAC,IAAI,CAAC,KAAK,CAAC,CAAC;SACpB;KACF;AACH,CAAC","sourcesContent":["/**\n * Pick the value from an array.\n */\nexport type PickValue<T> = T extends ReadonlyArray<any>\n  ? {\n      [K in Extract<keyof T, number>]: PickValue<T[K]>;\n    }[number]\n  : T;\n\n/**\n * Flatten an `ArrayLike` object in TypeScript.\n */\nexport type FlatArray<T extends ArrayLike<any>> = Array<PickValue<T[number]>>;\n\n/**\n * Flatten an array indefinitely.\n */\nexport function flatten<T extends ArrayLike<any>>(array: T): FlatArray<T> {\n  const result: FlatArray<T> = [];\n  $flatten<T>(array, result);\n  return result;\n}\n\n/**\n * Internal flatten function recursively passes `result`.\n */\nfunction $flatten<T extends ArrayLike<any>>(\n  array: T,\n  result: FlatArray<T>\n): void {\n  for (let i = 0; i < array.length; i++) {\n    const value = array[i];\n\n    if (Array.isArray(value)) {\n      $flatten(value as any, result);\n    } else {\n      result.push(value);\n    }\n  }\n}\n"]}
\ No newline at end of file
diff --git a/test/build/custom-modules-dir/node_modules/array-flatten/dist/index.spec.d.ts b/test/build/custom-modules-dir/node_modules/array-flatten/dist/index.spec.d.ts
new file mode 100644
index 0000000000..cb0ff5c3b5
--- /dev/null
+++ b/test/build/custom-modules-dir/node_modules/array-flatten/dist/index.spec.d.ts
@@ -0,0 +1 @@
+export {};
diff --git a/test/build/custom-modules-dir/node_modules/array-flatten/dist/index.spec.js b/test/build/custom-modules-dir/node_modules/array-flatten/dist/index.spec.js
new file mode 100644
index 0000000000..b5bf9b1d44
--- /dev/null
+++ b/test/build/custom-modules-dir/node_modules/array-flatten/dist/index.spec.js
@@ -0,0 +1,44 @@
+"use strict";
+Object.defineProperty(exports, "__esModule", { value: true });
+var ts_expect_1 = require("ts-expect");
+var index_1 = require("./index");
+describe("flatten", function () {
+    it("should flatten an array", function () {
+        var result = index_1.flatten([1, [2, [3, [4, [5]]], 6, [[7], 8], 9], 10]);
+        ts_expect_1.expectType(result);
+        expect(result).toStrictEqual([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);
+    });
+    it("should work with array-like", function () {
+        var result = index_1.flatten("test");
+        ts_expect_1.expectType(result);
+        expect(result).toStrictEqual(["t", "e", "s", "t"]);
+    });
+    it("should work with readonly array", function () {
+        var input = [1, [2, [3, [4]]]];
+        var result = index_1.flatten(input);
+        ts_expect_1.expectType(result);
+        expect(result).toStrictEqual([1, 2, 3, 4]);
+    });
+    it("should work with arguments", function () {
+        var input = (function () {
+            return arguments;
+        })();
+        var result = index_1.flatten(input);
+        ts_expect_1.expectType(result);
+        expect(result).toStrictEqual([]);
+    });
+    it("should work with mixed types", function () {
+        var fn = function (x) { return x; };
+        var input = [1, ["test", [fn, [true]]]];
+        var result = index_1.flatten(input);
+        ts_expect_1.expectType(result);
+        expect(result).toStrictEqual([1, "test", fn, true]);
+    });
+    it("should work with tuples", function () {
+        var input = [1, [1, 2], [3]];
+        var result = index_1.flatten(input);
+        ts_expect_1.expectType(result);
+        expect(result).toStrictEqual([1, 1, 2, 3]);
+    });
+});
+//# sourceMappingURL=index.spec.js.map
\ No newline at end of file
diff --git a/test/build/custom-modules-dir/node_modules/array-flatten/dist/index.spec.js.map b/test/build/custom-modules-dir/node_modules/array-flatten/dist/index.spec.js.map
new file mode 100644
index 0000000000..4d10f1dd36
--- /dev/null
+++ b/test/build/custom-modules-dir/node_modules/array-flatten/dist/index.spec.js.map
@@ -0,0 +1 @@
+{"version":3,"file":"index.spec.js","sourceRoot":"","sources":["../src/index.spec.ts"],"names":[],"mappings":";;AAAA,uCAAuC;AACvC,iCAAkC;AAElC,QAAQ,CAAC,SAAS,EAAE;IAClB,EAAE,CAAC,yBAAyB,EAAE;QAC5B,IAAM,MAAM,GAAG,eAAO,CAAC,CAAC,CAAC,EAAE,CAAC,CAAC,EAAE,CAAC,CAAC,EAAE,CAAC,CAAC,EAAE,CAAC,CAAC,CAAC,CAAC,CAAC,EAAE,CAAC,EAAE,CAAC,CAAC,CAAC,CAAC,EAAE,CAAC,CAAC,EAAE,CAAC,CAAC,EAAE,EAAE,CAAC,CAAC,CAAC;QAEpE,sBAAU,CAAW,MAAM,CAAC,CAAC;QAE7B,MAAM,CAAC,MAAM,CAAC,CAAC,aAAa,CAAC,CAAC,CAAC,EAAE,CAAC,EAAE,CAAC,EAAE,CAAC,EAAE,CAAC,EAAE,CAAC,EAAE,CAAC,EAAE,CAAC,EAAE,CAAC,EAAE,EAAE,CAAC,CAAC,CAAC;IAChE,CAAC,CAAC,CAAC;IAEH,EAAE,CAAC,6BAA6B,EAAE;QAChC,IAAM,MAAM,GAAG,eAAO,CAAC,MAAM,CAAC,CAAC;QAE/B,sBAAU,CAAW,MAAM,CAAC,CAAC;QAE7B,MAAM,CAAC,MAAM,CAAC,CAAC,aAAa,CAAC,CAAC,GAAG,EAAE,GAAG,EAAE,GAAG,EAAE,GAAG,CAAC,CAAC,CAAC;IACrD,CAAC,CAAC,CAAC;IAEH,EAAE,CAAC,iCAAiC,EAAE;QACpC,IAAM,KAAK,GAAG,CAAC,CAAC,EAAE,CAAC,CAAC,EAAE,CAAC,CAAC,EAAE,CAAC,CAAC,CAAC,CAAC,CAAC,CAAU,CAAC;QAC1C,IAAM,MAAM,GAAG,eAAO,CAAC,KAAK,CAAC,CAAC;QAE9B,sBAAU,CAAoB,MAAM,CAAC,CAAC;QAEtC,MAAM,CAAC,MAAM,CAAC,CAAC,aAAa,CAAC,CAAC,CAAC,EAAE,CAAC,EAAE,CAAC,EAAE,CAAC,CAAC,CAAC,CAAC;IAC7C,CAAC,CAAC,CAAC;IAEH,EAAE,CAAC,4BAA4B,EAAE;QAC/B,IAAM,KAAK,GAAG,CAAC;YACb,OAAO,SAAS,CAAC;QACnB,CAAC,CAAC,EAAE,CAAC;QACL,IAAM,MAAM,GAAG,eAAO,CAAC,KAAK,CAAC,CAAC;QAE9B,sBAAU,CAAQ,MAAM,CAAC,CAAC;QAE1B,MAAM,CAAC,MAAM,CAAC,CAAC,aAAa,CAAC,EAAE,CAAC,CAAC;IACnC,CAAC,CAAC,CAAC;IAEH,EAAE,CAAC,8BAA8B,EAAE;QACjC,IAAM,EAAE,GAAG,UAAC,CAAS,IAAK,OAAA,CAAC,EAAD,CAAC,CAAC;QAC5B,IAAM,KAAK,GAAG,CAAC,CAAC,EAAE,CAAC,MAAM,EAAE,CAAC,EAAE,EAAE,CAAC,IAAI,CAAC,CAAC,CAAC,CAAC,CAAC;QAC1C,IAAM,MAAM,GAAG,eAAO,CAAC,KAAK,CAAC,CAAC;QAE9B,sBAAU,CAA4C,MAAM,CAAC,CAAC;QAE9D,MAAM,CAAC,MAAM,CAAC,CAAC,aAAa,CAAC,CAAC,CAAC,EAAE,MAAM,EAAE,EAAE,EAAE,IAAI,CAAC,CAAC,CAAC;IACtD,CAAC,CAAC,CAAC;IAEH,EAAE,CAAC,yBAAyB,EAAE;QAC5B,IAAM,KAAK,GAAyC,CAAC,CAAC,EAAE,CAAC,CAAC,EAAE,CAAC,CAAC,EAAE,CAAC,CAAC,CAAC,CAAC,CAAC;QACrE,IAAM,MAAM,GAAG,eAAO,CAAC,KAAK,CAAC,CAAC;QAE9B,sBAAU,CAAW,MAAM,CAAC,CAAC;QAE7B,MAAM,CAAC,MAAM,CAAC,CAAC,aAAa,CAAC,CAAC,CAAC,EAAE,CAAC,EAAE,CAAC,EAAE,CAAC,CAAC,CAAC,CAAC;IAC7C,CAAC,CAAC,CAAC;AACL,CAAC,CAAC,CAAC","sourcesContent":["import { expectType } from \"ts-expect\";\nimport { flatten } from \"./index\";\n\ndescribe(\"flatten\", () => {\n  it(\"should flatten an array\", () => {\n    const result = flatten([1, [2, [3, [4, [5]]], 6, [[7], 8], 9], 10]);\n\n    expectType<number[]>(result);\n\n    expect(result).toStrictEqual([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);\n  });\n\n  it(\"should work with array-like\", () => {\n    const result = flatten(\"test\");\n\n    expectType<string[]>(result);\n\n    expect(result).toStrictEqual([\"t\", \"e\", \"s\", \"t\"]);\n  });\n\n  it(\"should work with readonly array\", () => {\n    const input = [1, [2, [3, [4]]]] as const;\n    const result = flatten(input);\n\n    expectType<(1 | 2 | 3 | 4)[]>(result);\n\n    expect(result).toStrictEqual([1, 2, 3, 4]);\n  });\n\n  it(\"should work with arguments\", () => {\n    const input = (function() {\n      return arguments;\n    })();\n    const result = flatten(input);\n\n    expectType<any[]>(result);\n\n    expect(result).toStrictEqual([]);\n  });\n\n  it(\"should work with mixed types\", () => {\n    const fn = (x: string) => x;\n    const input = [1, [\"test\", [fn, [true]]]];\n    const result = flatten(input);\n\n    expectType<(number | string | boolean | typeof fn)[]>(result);\n\n    expect(result).toStrictEqual([1, \"test\", fn, true]);\n  });\n\n  it(\"should work with tuples\", () => {\n    const input: [number, [number, number], [number]] = [1, [1, 2], [3]];\n    const result = flatten(input);\n\n    expectType<number[]>(result);\n\n    expect(result).toStrictEqual([1, 1, 2, 3]);\n  });\n});\n"]}
\ No newline at end of file
diff --git a/test/build/custom-modules-dir/node_modules/array-flatten/package.json b/test/build/custom-modules-dir/node_modules/array-flatten/package.json
new file mode 100644
index 0000000000..5a72db5154
--- /dev/null
+++ b/test/build/custom-modules-dir/node_modules/array-flatten/package.json
@@ -0,0 +1,126 @@
+{
+  "_from": "array-flatten@^3.0.0",
+  "_id": "array-flatten@3.0.0",
+  "_inBundle": false,
+  "_integrity": "sha512-zPMVc3ZYlGLNk4mpK1NzP2wg0ml9t7fUgDsayR5Y5rSzxQilzR9FGu/EH2jQOcKSAeAfWeylyW8juy3OkWRvNA==",
+  "_location": "/array-flatten",
+  "_phantomChildren": {},
+  "_requested": {
+    "type": "range",
+    "registry": true,
+    "raw": "array-flatten@^3.0.0",
+    "name": "array-flatten",
+    "escapedName": "array-flatten",
+    "rawSpec": "^3.0.0",
+    "saveSpec": null,
+    "fetchSpec": "^3.0.0"
+  },
+  "_requiredBy": [
+    "/"
+  ],
+  "_resolved": "https://registry.npmjs.org/array-flatten/-/array-flatten-3.0.0.tgz",
+  "_shasum": "6428ca2ee52c7b823192ec600fa3ed2f157cd541",
+  "_spec": "array-flatten@^3.0.0",
+  "_where": "/Users/dan/fork/snowpack/test/build/custom-modules-dir",
+  "author": {
+    "name": "Blake Embrey",
+    "email": "hello@blakeembrey.com",
+    "url": "http://blakeembrey.me"
+  },
+  "bugs": {
+    "url": "https://github.com/blakeembrey/array-flatten/issues"
+  },
+  "bundleDependencies": false,
+  "deprecated": false,
+  "description": "Flatten nested arrays",
+  "devDependencies": {
+    "@size-limit/preset-small-lib": "^2.2.1",
+    "@types/jest": "^24.0.23",
+    "@types/node": "^12.12.11",
+    "benchmarked": "^2.0.0",
+    "husky": "^3.1.0",
+    "jest": "^24.9.0",
+    "lint-staged": "^9.4.3",
+    "prettier": "^1.19.1",
+    "ts-expect": "^1.1.0",
+    "ts-jest": "^24.1.0",
+    "tslint": "^5.20.1",
+    "tslint-config-prettier": "^1.18.0",
+    "tslint-config-standard": "^9.0.0",
+    "typescript": "^3.7.2"
+  },
+  "files": [
+    "dist/",
+    "dist.es2015/",
+    "LICENSE"
+  ],
+  "homepage": "https://github.com/blakeembrey/array-flatten",
+  "husky": {
+    "hooks": {
+      "pre-commit": "lint-staged"
+    }
+  },
+  "jest": {
+    "roots": [
+      "<rootDir>/src/"
+    ],
+    "transform": {
+      "\\.tsx?$": "ts-jest"
+    },
+    "testRegex": "(/__tests__/.*|\\.(test|spec))\\.(tsx?|jsx?)$",
+    "moduleFileExtensions": [
+      "ts",
+      "tsx",
+      "js",
+      "jsx",
+      "json",
+      "node"
+    ]
+  },
+  "jsnext:main": "dist.es2015/index.js",
+  "keywords": [
+    "array",
+    "flatten",
+    "arguments",
+    "depth",
+    "fast",
+    "for"
+  ],
+  "license": "MIT",
+  "lint-staged": {
+    "*.{js,json,css,md}": [
+      "npm run prettier",
+      "git add"
+    ]
+  },
+  "main": "dist/index.js",
+  "module": "dist.es2015/index.js",
+  "name": "array-flatten",
+  "publishConfig": {
+    "access": "public"
+  },
+  "repository": {
+    "type": "git",
+    "url": "git://github.com/blakeembrey/array-flatten.git"
+  },
+  "scripts": {
+    "benchmark": "node benchmark",
+    "build": "rimraf dist/ dist.es2015/ && tsc && tsc -P tsconfig.es2015.json",
+    "format": "npm run prettier -- \"{.,src/**,benchmark/**}/*.{js,ts}\"",
+    "lint": "tslint \"src/**/*\" --project tsconfig.json",
+    "prepare": "npm run build",
+    "prettier": "prettier --write",
+    "size": "size-limit",
+    "specs": "jest --coverage",
+    "test": "npm run build && npm run lint && npm run specs && npm run size"
+  },
+  "sideEffects": false,
+  "size-limit": [
+    {
+      "path": "dist/index.js",
+      "limit": "100 B"
+    }
+  ],
+  "typings": "dist/index.d.ts",
+  "version": "3.0.0"
+}
diff --git a/test/build/custom-modules-dir/package-lock.json b/test/build/custom-modules-dir/package-lock.json
new file mode 100644
index 0000000000..5fc0d2ff4d
--- /dev/null
+++ b/test/build/custom-modules-dir/package-lock.json
@@ -0,0 +1,11 @@
+{
+  "requires": true,
+  "lockfileVersion": 1,
+  "dependencies": {
+    "array-flatten": {
+      "version": "3.0.0",
+      "resolved": "https://registry.npmjs.org/array-flatten/-/array-flatten-3.0.0.tgz",
+      "integrity": "sha512-zPMVc3ZYlGLNk4mpK1NzP2wg0ml9t7fUgDsayR5Y5rSzxQilzR9FGu/EH2jQOcKSAeAfWeylyW8juy3OkWRvNA=="
+    }
+  }
+}
diff --git a/test/build/custom-modules-dir/package.json b/test/build/custom-modules-dir/package.json
new file mode 100644
index 0000000000..0e74921042
--- /dev/null
+++ b/test/build/custom-modules-dir/package.json
@@ -0,0 +1,14 @@
+{
+  "scripts": {
+    "TEST": "node ../../../pkg/dist-node/index.bin.js build"
+  },
+  "snowpack": {
+    "scripts": {
+      "mount:src": "mount src --to /_dist_",
+      "mount:web_modules": "mount $WEB_MODULES --to /my_modules"
+    }
+  },
+  "dependencies": {
+    "array-flatten": "^3.0.0"
+  }
+}
diff --git a/test/build/custom-modules-dir/src/index.js b/test/build/custom-modules-dir/src/index.js
new file mode 100644
index 0000000000..2f851626fa
--- /dev/null
+++ b/test/build/custom-modules-dir/src/index.js
@@ -0,0 +1,5 @@
+import {flatten} from 'array-flatten';
+
+export default function doNothing() {
+  // I do nothing 🎉
+}