Fix missing concurrency check in [].forEach.

sdk/lib/core/list.dart

 // Copyright (c) 2012, 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.
 
 part of dart.core;
 
 /**
  * A [List] is an indexable collection with a length.
  *
  * A `List` implementation can choose not to support all methods
@@ skipping 15 matching lines @@
  *    fixedLengthList.add(499);  // throws
  *    fixedLengthList[0] = 87;
  *    var growableList = [1, 2];
  *    growableList.length = 0;
  *    growableList.add(499);
  *    growableList[0] = 87;
  *    var unmodifiableList = const [1, 2];
  *    unmodifiableList.length = 0;  // throws.
  *    unmodifiableList.add(499);  // throws
  *    unmodifiableList[0] = 87;  // throws.
+ *
+ * Lists are [Iterable].
+ * List iteration iterates over values in index order.
+ * Changing the values will not affect iteration,
+ * but changing the valid indices -
+ * that is, changing the list's length -
+ * between iteration steps
+ * will cause a [ConcurrentModificationError].
+ * This means that only growable lists can throw [ConcurrentModificationError].
+ * If the length changes temporarily
+ * and is restored before continuing the iteration,
+ * the iterator will not detect it.

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

Right. So why add the check?

  */
 abstract class List<E> implements Iterable<E> {
   /**
    * Creates a list of the given [length].
    *
    * The list is a fixed-length list if [length] is provided, and an empty
    * growable list if [length] is omitted.
    */
   external factory List([int length]);
 
@@ skipping 288 matching lines @@
   void replaceRange(int start, int end, Iterable<E> iterable);
 
   /**
    * Returns an unmodifiable [Map] view of `this`.
    *
    * It has the indices of this list as keys, and the corresponding elements
    * as values.
    */
   Map<int, E> asMap();
 }