Revert "Change some platform libraries to use `part of URI`."

sdk/lib/async/deferred_load.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.
 
-part of "dart:async";
+part of dart.async;
 
 /**
  * Indicates that loading of [libraryName] is deferred.
  *
  * This class is obsolete. Instead use the syntax:
  * import "library.dart" deferred as prefix;
  */
 @Deprecated("Dart sdk v. 1.8")
 class DeferredLibrary {
   final String libraryName;
@@ skipping 11 matching lines @@
 }
 
 /**
  * Thrown when a deferred library fails to load.
  */
 class DeferredLoadException implements Exception {
   DeferredLoadException(this._s);
   String toString() => "DeferredLoadException: '$_s'";
   final String _s;
 }