Move Iterable implementation to collection.

tests/language/factory2_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.
 
-abstract class Link<T> extends Iterable<T> {
+import "dart:collection";
+
+abstract class Link<T> extends IterableBase<T> {
   // does not match constructor for LinkFactory
   factory Link(T head, [Link<T> tail]) = LinkFactory<T>; /// static type warning
   Link<T> prepend(T element);
 }
 
 abstract class EmptyLink<T> extends Link<T> {
   const factory EmptyLink() = LinkTail<T>;
 }
 
 class LinkFactory<T> {
@@ skipping 25 matching lines @@
 }
 
 main() {
   new Fisk();
   // instantiation of abstract class
   new EmptyLink<String>().prepend('hest'); /// static type warning
   // instantiation of abstract class
   const EmptyLink<String>().prepend('fisk'); /// static type warning
 }