diff --git a/packages/angular_devkit/build_optimizer/src/build-optimizer/build-optimizer.ts b/packages/angular_devkit/build_optimizer/src/build-optimizer/build-optimizer.ts
index b4fb41f460..20d9ab0f76 100644
--- a/packages/angular_devkit/build_optimizer/src/build-optimizer/build-optimizer.ts
+++ b/packages/angular_devkit/build_optimizer/src/build-optimizer/build-optimizer.ts
@@ -90,18 +90,7 @@ export function buildOptimizer(options: BuildOptimizerOptions): TransformJavascr
   // Determine which transforms to apply.
   const getTransforms = [];
 
-  if (testWrapEnums(content)) {
-    getTransforms.push(getWrapEnumsTransformer);
-  }
-
-  if (testImportTslib(content)) {
-    getTransforms.push(getImportTslibTransformer);
-  }
-
-  if (testPrefixClasses(content)) {
-    getTransforms.push(getPrefixClassesTransformer);
-  }
-
+  let typeCheck = false;
   if (options.isSideEffectFree || originalFilePath && isKnownSideEffectFree(originalFilePath)) {
     getTransforms.push(
       // getPrefixFunctionsTransformer is rather dangerous, apply only to known pure es5 modules.
@@ -112,11 +101,29 @@ export function buildOptimizer(options: BuildOptimizerOptions): TransformJavascr
       getScrubFileTransformer,
       getFoldFileTransformer,
     );
+    typeCheck = true;
   } else if (testScrubFile(content)) {
+    // Always test as these require the type checker
     getTransforms.push(
       getScrubFileTransformer,
       getFoldFileTransformer,
     );
+    typeCheck = true;
+  }
+
+  // tests are not needed for fast path
+  const ignoreTest = !options.emitSourceMap && !typeCheck;
+
+  if (ignoreTest || testPrefixClasses(content)) {
+    getTransforms.unshift(getPrefixClassesTransformer);
+  }
+
+  if (ignoreTest || testImportTslib(content)) {
+    getTransforms.unshift(getImportTslibTransformer);
+  }
+
+  if (ignoreTest || testWrapEnums(content)) {
+    getTransforms.unshift(getWrapEnumsTransformer);
   }
 
   const transformJavascriptOpts: TransformJavascriptOptions = {
@@ -126,6 +133,7 @@ export function buildOptimizer(options: BuildOptimizerOptions): TransformJavascr
     emitSourceMap: options.emitSourceMap,
     strict: options.strict,
     getTransforms,
+    typeCheck,
   };
 
   return transformJavascript(transformJavascriptOpts);
diff --git a/packages/angular_devkit/build_optimizer/src/build-optimizer/build-optimizer_spec.ts b/packages/angular_devkit/build_optimizer/src/build-optimizer/build-optimizer_spec.ts
index b73072d307..588f0c8b5d 100644
--- a/packages/angular_devkit/build_optimizer/src/build-optimizer/build-optimizer_spec.ts
+++ b/packages/angular_devkit/build_optimizer/src/build-optimizer/build-optimizer_spec.ts
@@ -88,17 +88,6 @@ describe('build-optimizer', () => {
       });
     });
 
-    it('doesn\'t process files without decorators/ctorParameters/outside Angular', () => {
-      const input = tags.oneLine`
-        var Clazz = (function () { function Clazz() { } return Clazz; }());
-        ${staticProperty}
-      `;
-
-      const boOutput = buildOptimizer({ content: input });
-      expect(boOutput.content).toBeFalsy();
-      expect(boOutput.emitSkipped).toEqual(true);
-    });
-
     it('supports es2015 modules', () => {
       // prefix-functions would add PURE_IMPORTS_START and PURE to the super call.
       // This test ensures it isn't applied to es2015 modules.
diff --git a/packages/angular_devkit/build_optimizer/src/helpers/transform-javascript.ts b/packages/angular_devkit/build_optimizer/src/helpers/transform-javascript.ts
index e7adc84a5b..8cb1ac8ae6 100644
--- a/packages/angular_devkit/build_optimizer/src/helpers/transform-javascript.ts
+++ b/packages/angular_devkit/build_optimizer/src/helpers/transform-javascript.ts
@@ -15,7 +15,8 @@ export interface TransformJavascriptOptions {
   outputFilePath?: string;
   emitSourceMap?: boolean;
   strict?: boolean;
-  getTransforms: Array<(program: ts.Program) => ts.TransformerFactory<ts.SourceFile>>;
+  typeCheck?: boolean;
+  getTransforms: Array<(program?: ts.Program) => ts.TransformerFactory<ts.SourceFile>>;
 }
 
 export interface TransformJavascriptOutput {
@@ -24,6 +25,42 @@ export interface TransformJavascriptOutput {
   emitSkipped: boolean;
 }
 
+interface DiagnosticSourceFile extends ts.SourceFile {
+  readonly parseDiagnostics?: ReadonlyArray<ts.Diagnostic>;
+}
+
+function validateDiagnostics(diagnostics: ReadonlyArray<ts.Diagnostic>, strict?: boolean): boolean {
+  // Print error diagnostics.
+  const checkDiagnostics = (diagnostics: ReadonlyArray<ts.Diagnostic>) => {
+    if (diagnostics && diagnostics.length > 0) {
+      let errors = '';
+      errors = errors + '\n' + ts.formatDiagnostics(diagnostics, {
+        getCurrentDirectory: () => ts.sys.getCurrentDirectory(),
+        getNewLine: () => ts.sys.newLine,
+        getCanonicalFileName: (f: string) => f,
+      });
+
+      return errors;
+    }
+  };
+
+  const hasError = diagnostics.some(diag => diag.category === ts.DiagnosticCategory.Error);
+  if (hasError) {
+    // Throw only if we're in strict mode, otherwise return original content.
+    if (strict) {
+      throw new Error(`
+        TS failed with the following error messages:
+
+        ${checkDiagnostics(diagnostics)}
+      `);
+    } else {
+      return false;
+    }
+  }
+
+  return true;
+}
+
 export function transformJavascript(
   options: TransformJavascriptOptions,
 ): TransformJavascriptOutput {
@@ -46,23 +83,68 @@ export function transformJavascript(
     };
   }
 
-  // Print error diagnostics.
-  const checkDiagnostics = (diagnostics: ReadonlyArray<ts.Diagnostic>) => {
-    if (diagnostics && diagnostics.length > 0) {
-      let errors = '';
-      errors = errors + '\n' + ts.formatDiagnostics(diagnostics, {
-        getCurrentDirectory: () => ts.sys.getCurrentDirectory(),
-        getNewLine: () => ts.sys.newLine,
-        getCanonicalFileName: (f: string) => f,
-      });
+  const allowFastPath = options.typeCheck === false && !emitSourceMap;
+  const outputs = new Map<string, string>();
+  const tempFilename = 'bo-default-file.js';
+  const tempSourceFile = ts.createSourceFile(
+    tempFilename,
+    content,
+    ts.ScriptTarget.Latest,
+    allowFastPath,
+  );
+  const parseDiagnostics = (tempSourceFile as DiagnosticSourceFile).parseDiagnostics;
 
-      return errors;
-    }
+  const tsOptions: ts.CompilerOptions = {
+    // We target latest so that there is no downleveling.
+    target: ts.ScriptTarget.Latest,
+    isolatedModules: true,
+    suppressOutputPathCheck: true,
+    allowNonTsExtensions: true,
+    noLib: true,
+    noResolve: true,
+    sourceMap: emitSourceMap,
+    inlineSources: emitSourceMap,
+    inlineSourceMap: false,
   };
 
-  const outputs = new Map<string, string>();
-  const tempFilename = 'bo-default-file.js';
-  const tempSourceFile = ts.createSourceFile(tempFilename, content, ts.ScriptTarget.Latest);
+  if (allowFastPath && parseDiagnostics) {
+    if (!validateDiagnostics(parseDiagnostics, strict)) {
+      return {
+        content: null,
+        sourceMap: null,
+        emitSkipped: true,
+      };
+    }
+
+    const transforms = getTransforms.map((getTf) => getTf(undefined));
+
+    const result = ts.transform(tempSourceFile, transforms, tsOptions);
+    if (result.transformed.length === 0 || result.transformed[0] === tempSourceFile) {
+      return {
+        content: null,
+        sourceMap: null,
+        emitSkipped: true,
+      };
+    }
+
+    const printer = ts.createPrinter(
+      undefined,
+      {
+        onEmitNode: result.emitNodeWithNotification,
+        substituteNode: result.substituteNode,
+      },
+    );
+
+    const output = printer.printFile(result.transformed[0]);
+
+    result.dispose();
+
+    return {
+      content: output,
+      sourceMap: null,
+      emitSkipped: false,
+    };
+  }
 
   const host: ts.CompilerHost = {
     getSourceFile: (fileName) => {
@@ -83,39 +165,15 @@ export function transformJavascript(
     writeFile: (fileName, text) => outputs.set(fileName, text),
   };
 
-  const tsOptions: ts.CompilerOptions = {
-    // We target latest so that there is no downleveling.
-    target: ts.ScriptTarget.Latest,
-    isolatedModules: true,
-    suppressOutputPathCheck: true,
-    allowNonTsExtensions: true,
-    noLib: true,
-    noResolve: true,
-    sourceMap: emitSourceMap,
-    inlineSources: emitSourceMap,
-    inlineSourceMap: false,
-  };
-
   const program = ts.createProgram([tempFilename], tsOptions, host);
 
   const diagnostics = program.getSyntacticDiagnostics(tempSourceFile);
-  const hasError = diagnostics.some(diag => diag.category === ts.DiagnosticCategory.Error);
-
-  if (hasError) {
-    // Throw only if we're in strict mode, otherwise return original content.
-    if (strict) {
-      throw new Error(`
-        TS failed with the following error messages:
-
-        ${checkDiagnostics(diagnostics)}
-      `);
-    } else {
-      return {
-        content: null,
-        sourceMap: null,
-        emitSkipped: true,
-      };
-    }
+  if (!validateDiagnostics(diagnostics, strict)) {
+    return {
+      content: null,
+      sourceMap: null,
+      emitSkipped: true,
+    };
   }
 
   // We need the checker inside transforms.
diff --git a/packages/angular_devkit/build_optimizer/src/transforms/class-fold_spec.ts b/packages/angular_devkit/build_optimizer/src/transforms/class-fold_spec.ts
index 237c857ce9..8471e3630a 100644
--- a/packages/angular_devkit/build_optimizer/src/transforms/class-fold_spec.ts
+++ b/packages/angular_devkit/build_optimizer/src/transforms/class-fold_spec.ts
@@ -11,7 +11,7 @@ import { getFoldFileTransformer } from './class-fold';
 
 
 const transform = (content: string) => transformJavascript(
-  { content, getTransforms: [getFoldFileTransformer] }).content;
+  { content, getTransforms: [getFoldFileTransformer], typeCheck: true }).content;
 
 describe('class-fold', () => {
   it('folds static properties into class', () => {
diff --git a/packages/angular_devkit/build_optimizer/src/transforms/prefix-functions.ts b/packages/angular_devkit/build_optimizer/src/transforms/prefix-functions.ts
index d17331d375..a0c263f007 100644
--- a/packages/angular_devkit/build_optimizer/src/transforms/prefix-functions.ts
+++ b/packages/angular_devkit/build_optimizer/src/transforms/prefix-functions.ts
@@ -52,7 +52,6 @@ export function getPrefixFunctionsTransformer(): ts.TransformerFactory<ts.Source
 export function findTopLevelFunctions(parentNode: ts.Node): ts.Node[] {
   const topLevelFunctions: ts.Node[] = [];
 
-  let previousNode: ts.Node;
   function cb(node: ts.Node) {
     // Stop recursing into this branch if it's a function expression or declaration
     if (node.kind === ts.SyntaxKind.FunctionDeclaration
@@ -60,32 +59,25 @@ export function findTopLevelFunctions(parentNode: ts.Node): ts.Node[] {
       return;
     }
 
-    // We need to check specially for IIFEs formatted as call expressions inside parenthesized
-    // expressions: `(function() {}())` Their start pos doesn't include the opening paren
-    // and must be adjusted.
-    if (isIIFE(node)
-        && previousNode.kind === ts.SyntaxKind.ParenthesizedExpression
-        && node.parent
-        && !hasPureComment(node.parent)) {
-      topLevelFunctions.push(node.parent);
-    } else if ((node.kind === ts.SyntaxKind.CallExpression
-              || node.kind === ts.SyntaxKind.NewExpression)
-        && !hasPureComment(node)
-    ) {
-      topLevelFunctions.push(node);
+    let noPureComment = !hasPureComment(node);
+    let innerNode = node;
+    while (innerNode && ts.isParenthesizedExpression(innerNode)) {
+      innerNode = innerNode.expression;
+      noPureComment = noPureComment && !hasPureComment(innerNode);
     }
 
-    previousNode = node;
+    if (!innerNode) {
+      return;
+    }
 
-    ts.forEachChild(node, cb);
-  }
+    if (noPureComment) {
+      if (innerNode.kind === ts.SyntaxKind.CallExpression
+          || innerNode.kind === ts.SyntaxKind.NewExpression) {
+        topLevelFunctions.push(node);
+      }
+    }
 
-  function isIIFE(node: ts.Node): boolean {
-    return node.kind === ts.SyntaxKind.CallExpression
-      // This check was in the old ngo but it doesn't seem to make sense with the typings.
-      // TODO(filipesilva): ask Alex Rickabaugh about it.
-      // && !(<ts.CallExpression>node).expression.text
-      && (node as ts.CallExpression).expression.kind !== ts.SyntaxKind.PropertyAccessExpression;
+    ts.forEachChild(innerNode, cb);
   }
 
   ts.forEachChild(parentNode, cb);
@@ -116,6 +108,9 @@ export function findPureImports(parentNode: ts.Node): string[] {
 }
 
 function hasPureComment(node: ts.Node) {
+  if (!node) {
+    return false;
+  }
   const leadingComment = ts.getSyntheticLeadingComments(node);
 
   return leadingComment && leadingComment.some((comment) => comment.text === pureFunctionComment);
diff --git a/packages/angular_devkit/build_optimizer/src/transforms/scrub-file_spec.ts b/packages/angular_devkit/build_optimizer/src/transforms/scrub-file_spec.ts
index accfff1a56..d4d1295868 100644
--- a/packages/angular_devkit/build_optimizer/src/transforms/scrub-file_spec.ts
+++ b/packages/angular_devkit/build_optimizer/src/transforms/scrub-file_spec.ts
@@ -11,7 +11,7 @@ import { getScrubFileTransformer, testScrubFile } from './scrub-file';
 
 
 const transform = (content: string) => transformJavascript(
-  { content, getTransforms: [getScrubFileTransformer] }).content;
+  { content, getTransforms: [getScrubFileTransformer], typeCheck: true }).content;
 
 describe('scrub-file', () => {
   const clazz = 'var Clazz = (function () { function Clazz() { } return Clazz; }());';