Move Iterable implementation to collection.

tests/language/factory3_test.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.
 // VMOptions=--enable_checked_mode
 
-abstract class Link<T> extends Iterable<T> {
+import "dart:collection";
+
+abstract class Link<T> extends IterableBase<T> {
   factory Link(T head, [Link<T> tail]) = LinkEntry<T>;
   Link<T> prepend(T element);
 }
 
 abstract class EmptyLink<T> extends Link<T> {
   const factory EmptyLink() = LinkTail<T>;
 }
 
 class AbstractLink<T> implements Link<T> {
   const AbstractLink();
@@ skipping 18 matching lines @@
   Link<Fisk> nodes = const EmptyLink<Fisk>();
   final int id;
   Fisk(this.id);
   toString() => id.toString();
 }
 
 main() {
   new Fisk(0).nodes.prepend(new Fisk(1)).prepend(new Fisk(2));
 }