Add proper API for creating private symbols wrt a library.

tests/lib/mirrors/closures_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:expect/expect.dart';
 import 'stringify.dart';
 
 testIntercepted() {
   var instance = [];
-  var methodMirror = reflect(instance.toString);
-  String rest = ' in s(List)';
-  rest = ''; /// 01: ok
-  expect('Method(s(toString)$rest)', methodMirror.function);
-  Expect.equals('[]', methodMirror.apply([]).reflectee);
+  var closureMirror = reflect(instance.toString);
+  var methodMirror = closureMirror.function;
+  Expect.equals(#toString, methodMirror.simpleName);
+  Expect.equals('[]', closureMirror.apply([]).reflectee);
 }
 
 testNonIntercepted() {
   var closure = new Map().containsKey;
-  var mirror = reflect(closure);
-  String rest = ' in s(_HashMap)'; // Might become Map instead.
-  rest = ''; /// 01: ok
-  expect('Method(s(containsKey)$rest)', mirror.function);
-  Expect.isFalse(mirror.apply([7]).reflectee);
+  var closureMirror = reflect(closure);
+  var methodMirror = closureMirror.function;
+  Expect.equals(#containsKey, methodMirror.simpleName);
+  Expect.isFalse(closureMirror.apply([7]).reflectee);
 }
 
 main() {
   testIntercepted();
   testNonIntercepted();
 }