Enable "part of" for the async library.

sdk/lib/async/future.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.
 
-// part of dart.async;
+part of dart.async;
 
 /**
  * A [Future] is used to obtain a value sometime in the future.  Receivers of a
  * [Future] can obtain the value by passing a callback to [then]. For example:
  *
  *     Future<int> future = getFutureFromSomewhere();
  *     future.then((value) {
  *       print("I received the number $value");
  *     });
  *
@@ skipping 169 matching lines @@
    */
   static Future forEach(Iterable input, Future f(element)) {
     var iterator = input.iterator;
     Future nextElement(_) {
       if (!iterator.moveNext()) return new Future.immediate(null);
       return f(iterator.current).then(nextElement);
     }
     return nextElement(null);
   }
 }