Use Window.performance.now if available, with correct frequency value

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..0557c8a4855bdc4f8361ae928a847381ec2a3d2f 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 (1000 * JS('num', '#.webkitNow()', performance)).floor();

[Emily Fortuna, 2012/11/26 23:31:31]

This is the line that's different. The key thing I misunderstood previously is
even though window.performance.webkitNow() returns up to microsecond precision,
it still returns it in millisecond granularity. So, since now() is supposed to
only return an int, I multiply by 1000, and then take the floor.

+      }
+    }
+    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) {