diff --git a/src/dev/build/lib/__tests__/fs.js b/src/dev/build/lib/__tests__/fs.js
index 67b4b35c948cc4..3c0c5153420393 100644
--- a/src/dev/build/lib/__tests__/fs.js
+++ b/src/dev/build/lib/__tests__/fs.js
@@ -31,6 +31,8 @@ const FOO_TAR_PATH = resolve(FIXTURES, 'foo_dir.tar.gz');
 const BAR_TXT_PATH = resolve(FIXTURES, 'foo_dir/bar.txt');
 const WORLD_EXECUTABLE = resolve(FIXTURES, 'bin/world_executable');
 
+const isWindows = /^win/.test(process.platform);
+
 // get the mode of a file as a string, like 777, or 644,
 function getCommonMode(path) {
   return statSync(path).mode.toString(8).slice(-3);
@@ -184,7 +186,8 @@ describe('dev/build/lib/fs', () => {
     it('copies the mode of the source file', async () => {
       const destination = resolve(TMP, 'dest.txt');
       await copy(WORLD_EXECUTABLE, destination);
-      expect(getCommonMode(destination)).to.be('777');
+
+      expect(getCommonMode(destination)).to.be(isWindows ? '666' : '777');
     });
   });
 
@@ -225,8 +228,8 @@ describe('dev/build/lib/fs', () => {
         resolve(destination, 'foo_dir/foo'),
       ]);
 
-      expect(getCommonMode(resolve(destination, 'bin/world_executable'))).to.be('777');
-      expect(getCommonMode(resolve(destination, 'foo_dir/bar.txt'))).to.be('644');
+      expect(getCommonMode(resolve(destination, 'bin/world_executable'))).to.be(isWindows ? '666' : '777');
+      expect(getCommonMode(resolve(destination, 'foo_dir/bar.txt'))).to.be(isWindows ? '666' : '644');
     });
 
     it('applies select globs if specified, ignores dot files', async () => {
@@ -270,6 +273,7 @@ describe('dev/build/lib/fs', () => {
       expect(await read(resolve(destination, 'foo_dir/.bar'))).to.be('dotfile\n');
     });
 
+
     it('supports atime and mtime', async () => {
       const destination = resolve(TMP, 'a/b/c/d/e');
       const time = new Date(1425298511000);
diff --git a/src/dev/build/lib/fs.js b/src/dev/build/lib/fs.js
index 1c19a68d7136ae..d6809afded317e 100644
--- a/src/dev/build/lib/fs.js
+++ b/src/dev/build/lib/fs.js
@@ -129,9 +129,22 @@ export async function copyAll(sourceDir, destination, options = {}) {
       base: sourceDir,
       dot,
     }),
-    vfs.dest(destination),
-    ...(Boolean(time) ? [createMapStream(file => utimesAsync(file.path, time, time))] : []),
+    vfs.dest(destination)
   ]);
+
+  // we must update access and modified file times after the file copy
+  // has completed, otherwise the copy action can effect modify times.
+  if (Boolean(time)) {
+    await createPromiseFromStreams([
+      vfs.src(select, {
+        buffer: false,
+        cwd: destination,
+        base: destination,
+        dot,
+      }),
+      createMapStream(file => utimesAsync(file.path, time, time))
+    ]);
+  }
 }
 
 export async function getFileHash(path, algo) {