Move StopwatchImplementation from coreimpl to core as _StopwatchImpl.

lib/compiler/implementation/lib/core_patch.dart

Index: lib/compiler/implementation/lib/core_patch.dart
diff --git a/lib/compiler/implementation/lib/core_patch.dart b/lib/compiler/implementation/lib/core_patch.dart
index d458e743d8f866a58c57cb43d4ffeef35782f96d..d3f8e432c4dc6c1fa0f2b46a19af1b6bfe8ced43 100644
--- a/lib/compiler/implementation/lib/core_patch.dart
+++ b/lib/compiler/implementation/lib/core_patch.dart
@@ -141,3 +141,10 @@ patch class _DateImpl {
 
   patch int get weekday => Primitives.getWeekday(this);
 }
+
+
+// Patch for Stopwatch implementation.
+patch class _StopwatchImpl {
+  patch static int _frequency() => 1000;
+  patch static int _now() => Primitives.dateNow();
+}