Let stopwatch use window.performance.now when available. Fix for: http://code.google.com/p/dart/iss…

sdk/lib/_internal/compiler/implementation/lib/js_helper.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.
 
 library _js_helper;
 
 import 'dart:collection';
 
 part 'constant_map.dart';
 part 'native_helper.dart';
@@ skipping 517 matching lines @@
     if ((length is !int) || (length < 0)) {
       throw new ArgumentError(length);
     }
     var result = JS('=List', r'new Array(#)', length);
     JS('void', r'#.fixed$length = #', result, true);
     return result;
   }
 
   static num dateNow() => JS('num', r'Date.now()');
 
+  static num numMicroseconds() {
+    if (JS('bool', 'typeof window != "undefined" && window !== null')) {
+      var performance = JS('var', 'window.performance');
+      if (performance != null && 
+          JS('bool', 'typeof #.webkitNow == "function"', performance)) {
+        return JS('num', '#.webkitNow()', performance);
+      }
+    }
+    return 1000 * dateNow();
+  }
+
   // This is to avoid stack overflows due to very large argument arrays in
   // apply().  It fixes http://dartbug.com/6919
   static String _fromCharCodeApply(List<int> array) {
     String result = "";
     const kMaxApply = 500;
     int end = array.length;
     for (var i = 0; i < end; i += kMaxApply) {
       var subarray;
       if (end <= kMaxApply) {
         subarray = array;
@@ skipping 1044 matching lines @@
     var argument = typeInfo[i];
     if (argument == null) {
       argument = 'dynamic';
     }
     arguments.add(argument);
   }
   return '$className<$arguments>';
 }
 
 createRuntimeType(String name) => new TypeImpl(name);