Add proper API for creating private symbols wrt a library.

tests/lib/mirrors/intercepted_superclass_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.
+
+library test.intercepted_superclass_test;
+
+import 'dart:mirrors';
+import 'package:expect/expect.dart';
+
+check(ClassMirror cm){
+  Expect.isTrue(cm is ClassMirror);
+  Expect.isNotNull(cm);
+}
+
+main() {
+  check(reflect('').type.superclass);
+  check(reflect(1).type.superclass);
+  check(reflect(1.5).type.superclass);
+  check(reflect(true).type.superclass);
+  check(reflect(false).type.superclass);
+  check(reflect([]).type.superclass);
+
+  check(reflectClass(String).superclass);
+  check(reflectClass(int).superclass);
+  check(reflectClass(double).superclass);
+  check(reflectClass(num).superclass);
+  check(reflectClass(bool).superclass);
+  check(reflectClass(List).superclass);
+}