Convert most interfaces in dart:core to abstract classes.

lib/core/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 default StopwatchImplementation {
-
+abstract class Stopwatch {
   /**
    * Creates a [Stopwatch] in stopped state with a zero elapsed count.
    *
    * The following example shows how to start a [Stopwatch]
    * right after allocation.
    *
    *     Stopwatch stopwatch = new Stopwatch()..start();
    */
-  Stopwatch();
+  factory Stopwatch() => new StopwatchImplementation();
 
   /**
    * Starts the [Stopwatch]. The [elapsed] count is increasing monotonically.
    * If the [Stopwatch] has been stopped, then calling start again restarts it
    * without resetting the [elapsed] count.
    * If the [Stopwatch] is currently running, then calling start does nothing.
    */
   void start();
 
   /**
@@ skipping 28 matching lines @@
    * Returns the [elapsed] counter converted to milliseconds.
    */
   int elapsedInMs();
 
   /**
    * Returns the frequency of the elapsed counter in Hz.
    */
   int frequency();
 
 }