Add "mock" compilation feature to dart2js and use it to improve test coverage.

dart/frog/leg/lib/mock.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 things that Leg cannot read directly.
 
 // TODO(ahe): Remove this file.
 
 class AssertionError {}
 class TypeError extends AssertionError {}
 
 class FallThroughError {
   const FallThroughError();
   String toString() => "Switch case fall-through.";
 }
 
 // TODO(ahe): VM specfic exception?
 class InternalError {
   const InternalError(this._msg);
   String toString() => "InternalError: '${_msg}'";
   final String _msg;
 }
 
 // TODO(ahe): VM specfic exception?
 class StaticResolutionException implements Exception {}
 
 void assert(condition) {}
+
+interface ByteArray extends List default _InternalByteArray {
+  ByteArray(int length);
+}
+
+class _InternalByteArray {
+  factory _InternalByteArray(int length) {
+    throw new UnsupportedOperationException("new ByteArray($length)");
+  }
+}