Update stopwatch to use patching to share sources instead of exposing

runtime/lib/stopwatch_patch.dart

Index: runtime/lib/stopwatch_patch.dart
diff --git a/runtime/lib/stopwatch_patch.dart b/runtime/lib/stopwatch_patch.dart
new file mode 100644
index 0000000000000000000000000000000000000000..b81b8b0a7158dc9f077f5a05f7402e599ab3336d
--- /dev/null
+++ b/runtime/lib/stopwatch_patch.dart
@@ -0,0 +1,13 @@
+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// A VM patch of the stopwatch part of dart:core.
+
+patch class StopwatchImplementation {
+  // Returns the current clock tick.
+  /* patch */ static int _now() native "Stopwatch_now";
+
+  // Returns the frequency of clock ticks in Hz.
+  /* patch */ static int _frequency() native "Stopwatch_frequency";
+}