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

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

Index: sdk/lib/_internal/compiler/js_lib/js_helper.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/js_helper.dart b/sdk/lib/_internal/compiler/js_lib/js_helper.dart
index 84213c6024e095d0edc19213ca57844db22ee29c..8b7ac87d4345b065af328b70e83be27be2a2086c 100644
--- a/sdk/lib/_internal/compiler/js_lib/js_helper.dart
+++ b/sdk/lib/_internal/compiler/js_lib/js_helper.dart
@@ -1435,8 +1435,23 @@ 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);
+  }
 }
 
 /**