Fix missing concurrency check in [].forEach.

runtime/lib/growable_array.dart

Index: runtime/lib/growable_array.dart
diff --git a/runtime/lib/growable_array.dart b/runtime/lib/growable_array.dart
index 95594b60c81805cc6ed27cfdb59a5ab4f1ac1e47..f65380b68be6a79ab7cf3a89ec1840c875679073 100644
--- a/runtime/lib/growable_array.dart
+++ b/runtime/lib/growable_array.dart
@@ -238,10 +238,10 @@ class _GrowableObjectArray<T> implements List<T> {
   }
 
   void forEach(f(T element)) {
-    // TODO(srdjan): Use IterableMixinWorkaround.forEach(this, f);
-    // Accessing the list directly improves DeltaBlue performance by 25%.
+    int initialLength = length;
     for (int i = 0; i < length; i++) {
       f(this[i]);
+      if (length != initialLength) throw new ConcurrentModificationError(this);

[hausner, 2013/04/25 23:07:17]

This is not sufficient to detect modifications. The mutator could remove one
element and add one. That would go undetected with this check.

I think you need a counter in the array that gets incremented each time the list
is modified. Whether it is worth paying the price for detecting this is another
question.

[Ivan Posva, 2013/04/26 05:58:22]

I think the only concern is that length could change. Basically if you asked for
the length of this List, you'd expect that forEach is called the same number of
times.

     }
   }