Enable "part of" for the async library.

sdk/lib/async/async_error.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;
 
 /**
  * Error result of an asynchronous computation.
  */
 class AsyncError {
   /** The actual error thrown by the computation. */
   final Object error;
   /** Stack trace corresponding to the error, if available. */
   final Object stackTrace;
   /** Asynchronous error leading to this error, if error handling fails. */
@@ skipping 49 matching lines @@
         // directly so that we can pass the stack trace.
         throw error;
       });
     } catch (e) {
       // Unfortunately there is not much more we can do...
       reportError();
     }
   }
 }