Start implementing checked mode and tools support for using it.

lib/compiler/implementation/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 TypeError extends AssertionError {
+  final String msg;
+  const TypeError(String this.msg);
+  String toString() => msg;
+}
 
 class FallThroughError {
   const FallThroughError();
   String toString() => "Switch case fall-through.";
 }
 
 // TODO(ahe): VM specfic exception?
 class InternalError {
   const InternalError(this._msg);
   String toString() => "InternalError: '${_msg}'";
@@ skipping 13 matching lines @@
 class _InternalByteArray {
   factory Uint8List(int length) {
     throw new UnsupportedOperationException("new Uint8List($length)");
   }
 }
 
 // TODO(ahe): This definitely does not belong in the core library.
 void exit(int exitCode) {
   throw new UnsupportedOperationException("exit($exitCode)");
 }