Rewrite for-in loop as indexing loop for JavaScript indexable arrays

sdk/lib/_internal/compiler/js_lib/js_helper.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library _js_helper;
 
 import 'dart:_async_await_error_codes' as async_error_codes;
 
 import 'dart:_js_embedded_names' show
     DEFERRED_LIBRARY_URIS,
@@ skipping 1417 matching lines @@
 }
 
 throwRuntimeError(message) {
   throw new RuntimeError(message);
 }
 
 throwAbstractClassInstantiationError(className) {
   throw new AbstractClassInstantiationError(className);
 }
 
-throwConcurrentModificationError(collection) {
-  throw new ConcurrentModificationError(collection);
+// Can be called
+//
+//     throwConcurrentModificationError(a.length == startLength, a)
+//
+// or
+//
+//     s.length == startLength || throwConcurrentModificationError(a)
+//
+@NoInline() @NoSideEffects()
+throwConcurrentModificationError(sameLength, [collection]) {

[floitsch, 2015/04/16 19:22:53]

I would prefer a separate function which you then inline in the codegen.

[sra1, 2015/04/16 23:37:24]

Done.

+  if (collection == null) {
+    collection = sameLength;
+    sameLength = false;
+  }
+  if (true != sameLength) {
+    throw new ConcurrentModificationError(collection);
+  }
 }
 
 /**
  * Helper class for building patterns recognizing native type errors.
  */
 class TypeErrorDecoder {
   // Field names are private to help tree-shaking.
 
   /// A regular expression which matches is matched against an error message.
   final String _pattern;
@@ skipping 2529 matching lines @@
   // This is a function that will return a helper function that does the
   // iteration of the sync*.
   //
   // Each invocation should give a body with fresh state.
   final dynamic /* js function */ _outerHelper;
 
   SyncStarIterable(this._outerHelper);
 
   Iterator get iterator => new SyncStarIterator(JS('', '#()', _outerHelper));
 }