Make dartdoc compile.

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) {}
 
+// TODO(ahe): Not sure ByteArray belongs in the core library.
 interface ByteArray extends List default _InternalByteArray {
   ByteArray(int length);
 }
 
 class _InternalByteArray {
   factory _InternalByteArray(int length) {
     throw new UnsupportedOperationException("new ByteArray($length)");
   }
 }
+
+// TODO(ahe): This definitely does not belong in the core library.
+void exit(int exitCode) {
+  throw new UnsupportedOperationException("exit($exitCode)");
+}