Add TypeMirror.isSubtypeOf, TypeMirror.isAssignableTo, ClassMirror.isSubclassOf

tests/compiler/dart2js/mirrors/relation_assignable_test.dart

+// Copyright (c) 2013, 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.
+
+import "dart:mirrors";
+
+import "package:async_helper/async_helper.dart";
+
+import "mirrors_test_helper.dart";
+import "../../../lib/mirrors/relation_assignable_test.dart";
+
+main() {
+  asyncTest(() => analyze("relation_assignable_test.dart").
+      then((MirrorSystem mirrors) {
+    test(mirrors);
+  }));
+}