Let SyncStartIterable extend IterableBase instead of Iterable.

sdk/lib/_internal/compiler/js_lib/js_helper.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.
 
 library _js_helper;
 
 import 'dart:_async_await_error_codes' as async_error_codes;
 
 import 'dart:_js_embedded_names' show
     DEFERRED_LIBRARY_URIS,
@@ skipping 4067 matching lines @@
         return moveNext();
       }
     }
     return true;
   }
 }
 
 /// An Iterable corresponding to a sync* method.
 ///
 /// Each invocation of a sync* method will return a new instance of this class.
-class SyncStarIterable extends Iterable {
+class SyncStarIterable extends IterableBase {
   // This is a function that will return a helper function that does the
   // iteration of the sync*.
   //
   // Each invocation should give a body with fresh state.
   final dynamic /* js function */ _outerHelper;
 
   SyncStarIterable(this._outerHelper);
 
   Iterator get iterator => new SyncStarIterator(JS('', '#()', _outerHelper));
 }