Make IterableMixinWorkaround using classes return correctly typed Iterables.

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 31 matching lines @@
  * *NOTICE*: Forwarding just these four operations to a normal growable [List]
  * (as created by `new List()`) will give very bad performance for `add` and
  * `addAll` operations of `ListBase`. These operations are implemented by
  * increasing the length of the list by one for each `add` operation, and
  * repeatedly increasing the length of a growable list is not efficient.
  * To avoid this, either override 'add' and 'addAll' to also forward directly
  * to the growable list, or, if possible, use `DelegatingList` from
  * "package:collection/wrappers.dart" instead.
  */
 abstract class ListMixin<E> implements List<E> {
-
   // Iterable interface.
   Iterator<E> get iterator => new ListIterator<E>(this);
 
   E elementAt(int index) => this[index];
 
   void forEach(void action(E element)) {
     int length = this.length;
     for (int i = 0; i < length; i++) {
       action(this[i]);
       if (length != this.length) {
@@ skipping 439 matching lines @@
       for (E element in iterable) {
         this[index++] = element;
       }
     }
   }
 
   Iterable<E> get reversed => new ReversedListIterable(this);
 
   String toString() => IterableBase.iterableToFullString(this, '[', ']');
 }