Move Iterable implementation to collection.

tests/corelib/iterable_length_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.
 
+import "dart:collection";
 import "package:expect/expect.dart";
 
-class A extends Iterable {
+class A extends IterableBase {
   int count;
   A(this.count);
 
   Iterator get iterator {
     return new AIterator(count);
   }
 }
 
 class AIterator implements Iterator {
   int _count;
@@ skipping 16 matching lines @@
 
 main() {
   var a = new A(10);
   Expect.equals(10, a.length);
   a = new A(0);
   Expect.equals(0, a.length);
   a = new A(5);
   Expect.equals(5, a.map((e) => e + 1).length);
   Expect.equals(3, a.where((e) => e >= 3).length);
 }