Use shared implementation of Stopwatch.

dart/frog/leg/lib/mock.dart

Index: dart/frog/leg/lib/mock.dart
diff --git a/dart/frog/leg/lib/mock.dart b/dart/frog/leg/lib/mock.dart
index c10a1b605d1a821e7e85070bd10e126cd82cea80..518f98495e4fe0e2be139f1dc5fc814903a41683 100644
--- a/dart/frog/leg/lib/mock.dart
+++ b/dart/frog/leg/lib/mock.dart
@@ -23,51 +23,4 @@ class List<E> implements Iterable<E> {
   }
 }
 
-class Stopwatch {
-  double startMs;
-  double elapsedMs;
-
-  Stopwatch() {
-    elapsedMs = 0.0;
-  }
-
-  void start() {
-    if (startMs == null) {
-      startMs = Primitives.dateNow();
-    }
-  }
-
-  void stop() {
-    if (startMs == null) return;
-    elapsedMs += Primitives.dateNow() - startMs;
-    startMs = null;
-  }
-
-  void reset() {
-    elapsedMs = 0.0;
-    if (startMs == null) return;
-    startMs = Primitives.dateNow();
-  }
-
-  int elapsed() {
-    return elapsedInMs();
-  }
-
-  int elapsedInUs() {
-    return elapsedInMs() * 1000;
-  }
-
-  int elapsedInMs() {
-    if (startMs == null) {
-      return Primitives.mathFloor(elapsedMs);
-    } else {
-      return Primitives.mathFloor(elapsedMs + (Primitives.dateNow() - startMs));
-    }
-  }
-
-  int frequency() {
-    return 1000;
-  }
-}
-
 void assert(condition) {}