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

sdk/lib/async/timer.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 count-down timer that can be configured to fire once or repeatedly.
  *
  * The timer counts down from the specified duration to 0.
  * When the timer reaches 0, the timer invokes the specified callback function.
  * Use a periodic timer to repeatedly count down the same interval.
  *
  * A negative duration is treated the same as a duration of 0.
  * If the duration is statically known to be 0, consider using [run].
@@ skipping 86 matching lines @@
    * and the timer has not been canceled.
    *
    * A periodic timer is active if it has not been canceled.
    */
   bool get isActive;
 
   external static Timer _createTimer(Duration duration, void callback());
   external static Timer _createPeriodicTimer(
       Duration duration, void callback(Timer timer));
 }