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 279ee270849d8dd3f34dc94de87ee8f05a6af0d6..9f657752846616e032cd0f3119dff0987688df1a 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
@@ -535,6 +535,18 @@ class Primitives {
 
   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);