Extend Iterable instead of implementing it.

tests/compiler/dart2js_extra/for_in_test.dart

 // Copyright (c) 2011, 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.
 
 // Test foreach (aka. for-in) functionality.
 
 testIterator(List expect, Iterable input) {
   int i = 0;
   for (var value in input) {
     Expect.isTrue(i < expect.length);
     Expect.equals(expect[i], value);
     i += 1;
   }
   Expect.equals(expect.length, i);
 
   i = 0;
   var value2;
   for (value2 in input) {
     Expect.isTrue(i < expect.length);
     Expect.equals(expect[i], value2);
     i += 1;
   }
   Expect.equals(expect.length, i);
 }
 
-class MyIterable<T> implements Iterable<T> {
+class MyIterable<T> extends Iterable<T> {
   final List<T> values;
   MyIterable(List<T> values) : this.values = values;
   Iterator iterator() {
     return new MyListIterator(values);
   }
 }
 
 class MyListIterator<T> implements Iterator<T> {
   final List<T> values;
   int index;
@@ skipping 34 matching lines @@
 
   // Using the same variable (showing that the expression is evaluated
   // in the outer scope).
   int result = 0;
   var x = [1,2,3];
   for (var x in x) {
     result += x;
   }
   Expect.equals(6, result);
 }