Use shared implementation of Stopwatch.

dart/frog/leg/lib/mockimpl.dart

 // 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.
 
 // Mocks of classes and interfaces that Leg cannot read directly.
 
 // TODO(ahe): Remove this file.
 
-class JSSyntaxRegExp {}
-class StringBufferImpl {}
+class JSSyntaxRegExp implements RegExp {
+  JSSyntaxRegExp(String pattern,
+                 [bool multiLine = false,
+                  bool ignoreCase = false]) {
+    throw 'JSSyntaxRegExp is not implemented';
+  }
+}
+
+class StringBufferImpl {
+  StringBufferImpl() {
+    throw 'StringBufferImpl is not implemented';
+  }
+}
+
 class DateImplementation {}
 class ReceivePortFactory {}
 
 class StringBase {
   static String createFromCharCodes(List<int> charCodes) {
     throw "StringBase.createFromCharCodes is not implemented";
   }
 
   static String join(List<String> strings, String separator) {
     throw "StringBase.join is not implemented";
@@ skipping 54 matching lines @@
   }
 
   static num pow(num value, num exponent) {
     throw 'MathNatives.pow is not implemented';
   }
 
   static double random() {
     throw 'MathNatives.random is not implemented';
   }
 }