Add proper API for creating private symbols wrt a library.

sdk/lib/_internal/lib/mirrors_patch.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.
 
 // Patch library for dart:mirrors.
 
 import 'dart:_js_mirrors' as js;
 
 patch class MirrorSystem {
   patch static String getName(Symbol symbol) => js.getName(symbol);
+  patch static Symbol getSymbol(String name, [LibraryMirror lib]) {
+    throw new UnimplementedError("MirrorSystem.getSymbol not implemented");
+  }
 }
 
 patch MirrorSystem currentMirrorSystem() => js.currentJsMirrorSystem;
 
 patch Future<MirrorSystem> mirrorSystemOf(SendPort port) {
   throw new UnsupportedError("MirrorSystem not implemented");
 }
 
 patch InstanceMirror reflect(Object reflectee) => js.reflect(reflectee);
 
 patch ClassMirror reflectClass(Type key) {
   return js.reflectType(key).originalDeclaration;
 }