Add default implementation of toString for ListMixin.

sdk/lib/collection/list.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.
 
 part of dart.collection;
 
 /**
  * Abstract implementation of a list.
  *
  * All operations are defined in terms of `length`, `operator[]`,
@@ skipping 404 matching lines @@
     }
     for (int i = startIndex; i >= 0; i--) {
       if (this[i] == element) {
         return i;
       }
     }
     return -1;
   }
 
   Iterable<E> get reversed => new ReversedListIterable(this);
+
+  String toString() => ToString.iterableToString(this);
 }