Mock up all remaining core library implementation classes.

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 {}
 
 // TODO(ahe): VM specfic exception?
-class InternalError {}
+class InternalError {
+  const InternalError(this._msg);
+  String toString() => "InternalError: '${_msg}'";
+  final String _msg;
+}
 
 // TODO(ahe): VM specfic exception?
 class StaticResolutionException implements Exception {}
 
 class List<E> implements Iterable<E> {
   factory List([int length]) => Primitives.newList(length);
   factory List.from(Iterable<E> other) {
     throw 'List.from is not implemented';
   }
 }
 
 void assert(condition) {}