Enable "part of" for the async library.

sdk/lib/async/future_impl.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;
 
 deprecatedFutureValue(_FutureImpl future) =>
   future._isComplete ? future._resultOrListeners : null;
 
 
 class _CompleterImpl<T> implements Completer<T> {
   final Future<T> future;
   bool _isComplete = false;
 
   _CompleterImpl() : future = new _FutureImpl<T>();
@@ skipping 437 matching lines @@
   Future catchError(function(AsyncError error), {bool test(var error)}) {
     return _future.catchError(function, test: test);
   }
 
   Future whenComplete(void action()) {
     return _future.whenComplete(action);
   }
 
   Stream<T> asStream() => new Stream.fromFuture(this);
 }