Document that concurrent modification is bad, m'kay.

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;
 
 /**
  * An indexable collection of objects with a length.
  *
  * Subclasses of this class implement different kinds of lists.
@@ skipping 17 matching lines @@
  *     growableList.add(499);
  *     growableList[0] = 87;
  *
  * Lists are [Iterable]. Iteration occurs over values in index order. Changing
  * the values does not affect iteration, but changing the valid
  * indices—that is, changing the list's length—between iteration
  * steps causes 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
  * does not detect it.
+ *
+ * It is generally not allowed to modify the list's length (adding or removing
+ * elements) while an operation on the list is being performed,
+ * for example during a call to [forEach] or [sort].
+ * Changing the list's length while it is being iterated, either by iterating it
+ * directly or through iterating an `Iterable` that is backed by the list, will
+ * break the iteration.
  */
 abstract class List<E> implements Iterable<E>, EfficientLength {
   /**
    * Creates a list of the given length.
    *
    * The created list is fixed-length if [length] is provided.
    *
    *     List fixedLengthList = new List(3);
    *     fixedLengthList.length;     // 3
-         fixedLengthList.length = 1; // Error
+   *     fixedLengthList.length = 1; // Error
    *
    *
    * The list has length 0 and is growable if [length] is omitted.
    *
    *     List growableList = new List();
    *     growableList.length; // 0;
    *     growableList.length = 3;
    *
    * An error occurs if [length] is negative.
    */
@@ skipping 371 matching lines @@
    * as values. The `Map.keys` [Iterable] iterates the indices of this list
    * in numerical order.
    *
    *     List<String> words = ['fee', 'fi', 'fo', 'fum'];
    *     Map<int, String> map = words.asMap();
    *     map[0] + map[1];   // 'feefi';
    *     map.keys.toList(); // [0, 1, 2, 3]
    */
   Map<int, E> asMap();
 }