Make IterableMixinWorkaround using classes return correctly typed Iterables.

sdk/lib/internal/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._internal;
 
 /**
  * Mixin that throws on the length changing operations of [List].
  *
  * Intended to mix-in on top of [ListMixin] for fixed-length lists.
@@ skipping 277 matching lines @@
 
   /** This operation is not supported by an unmodifiable map. */
   void clear() {
     throw new UnsupportedError("Cannot modify an unmodifiable map");
   }
 
   /** This operation is not supported by an unmodifiable map. */
   void addAll(Map<int, E> other) {
     throw new UnsupportedError("Cannot modify an unmodifiable map");
   }
+
+  String toString() => Maps.mapToString(this);
 }
 
 class ReversedListIterable<E> extends ListIterable<E> {
   Iterable<E> _source;
   ReversedListIterable(this._source);
 
   int get length => _source.length;
 
   E elementAt(int index) => _source.elementAt(_source.length - 1 - index);
 }
@@ skipping 59 matching lines @@
  * or become empty or been otherwise modified.
  * It will still be a valid object, so references to it will not, e.g., crash
  * the runtime if accessed, but no promises are made wrt. its contents.
  *
  * This unspecified behavior is the reason the function is not exposed to
  * users. We allow the underlying implementation to make the most efficient
  * conversion, at the cost of leaving the original list in an unspecified
  * state.
  */
 external List makeListFixedLength(List growableList);