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() {
+    var theWindow = JS('var', 'window');

[Lasse Reichstein Nielsen, 2012/11/22 11:26:48]

This read will throw if 'window' is not there.
Consider using 
 bool windowExists = JS('bool', 'typeof window != "undefined" && window !==
null');
If that is true, then 'window.performance' is safe.

If this is slow, we can consider caching the function, or the fact that the
function exists, so we don't have to check every time. Too much overhead on a
function returning micro seconds seems counter productive :)

[ahe, 2012/11/22 12:54:46]

You don't have to call the variable theWindow.  We know that it is bad to use
the name "window" for a local variable and calls it something else.

+    if (theWindow != null) {
+      var performance = JS('var', 'window.performance');
+      if (performance != null && 
+          JS('bool', 'typeof #.webkitNow == "function"', performance)) {
+        return JS('num', '#.webkitNow()', performance);
+      }
+    }
+    return 1000 * dateNow();
+  }
+
   static String stringFromCharCodes(charCodes) {
     for (var i in charCodes) {
       if (i is !int) throw new ArgumentError(i);
     }
     return JS('String', r'String.fromCharCode.apply(#, #)', null, charCodes);
   }
 
   static String getTimeZoneName(receiver) {
     // When calling toString on a Date it will emit the timezone in parenthesis.
     // Example: "Wed May 16 2012 21:13:00 GMT+0200 (CEST)".
@@ skipping 988 matching lines @@
     return typeName == other.typeName;
   }
 }
 
 String getRuntimeTypeString(var object) {
   var typeInfo = JS('var', r'#.builtin$typeInfo', object);
   return JS('String', r'#.runtimeType', typeInfo);
 }
 
 createRuntimeType(String name) => new TypeImpl(name);