Updates the corelib to the new default specification for interfaces and new factory syntax

corelib/src/stopwatch.dart

 // Copyright (c) 2011, 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.
 
 /**
  * A simple [Stopwatch] interface to measure elapsed time.
  */
-interface Stopwatch factory StopwatchImplementation {
+interface Stopwatch default StopwatchImplementation {
 
   /**
    * Creates a [Stopwatch] in stopped state with a zero elapsed count.
    */
   Stopwatch();
 
   /**
    * Creates a [Stopwatch] in started state with a zero elapsed count.
    */
   Stopwatch.start();
@@ skipping 38 matching lines @@
    * Returns the [elapsed] counter converted to milliseconds.
    */
   int elapsedInMs();
 
   /**
    * Returns the frequency of the elapsed counter in Hz.
    */
   int frequency();
 
 }