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

sdk/lib/_internal/compiler/implementation/lib/core_patch.dart

Index: sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart b/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
index 21cf6e1cdc0ab1da24c328f97f139632d7ae5214..38b90d1e05f296c63fef903c9f9042eca897250b 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
@@ -148,8 +148,8 @@ patch class _DateImpl {
 
 // Patch for Stopwatch implementation.
 patch class _StopwatchImpl {
-  patch static int _frequency() => 1000;
-  patch static int _now() => Primitives.dateNow();
+  patch static int _frequency() => 1000000;
+  patch static int _now() => Primitives.numMicroseconds();
 }