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

Index: sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart b/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
index 161ebf1e5b89006302e41c8adcff9c1744840a3d..ad8d98c36270294e1cba181592e2c88446c1bcc2 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
@@ -535,6 +535,17 @@ class Primitives {
 
   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) {