Use shared implementation of Stopwatch.

dart/frog/leg/lib/mockimpl.dart

Index: dart/frog/leg/lib/mockimpl.dart
diff --git a/dart/frog/leg/lib/mockimpl.dart b/dart/frog/leg/lib/mockimpl.dart
index 959b67536159b3e93ecbb8e0b150fd53627f04e1..8d12cba89d0acd1907439270e9a6a2941df32a1a 100644
--- a/dart/frog/leg/lib/mockimpl.dart
+++ b/dart/frog/leg/lib/mockimpl.dart
@@ -6,8 +6,20 @@
 
 // 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 {}