Update Dart protoc plugin to latest SDK

test/reserved_names_test.dart

 #!/usr/bin/env 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 reserved_names_test;
 
 import 'dart:mirrors' as mirrors;
 
 import 'package:unittest/unittest.dart';
 
 import '../lib/protoc.dart' show MessageGenerator;
 
 import 'test_util.dart';
 
 void main() {
   test('testReservedNamesList', () {
     Set<String> names = new Set<String>();
-    fillConflictingNames(ClassMirror cls) {
+    fillConflictingNames(mirrors.ClassMirror cls) {
       String className = mirrors.MirrorSystem.getName(cls.simpleName);
       names.addAll(
           cls
           .constructors
           .values
           .where((ctor) => !ctor.isPrivate)
           .map((m) => mirrors.MirrorSystem.getName(m.simpleName))
           .map((n) => n.startsWith(className + '.')
                       ? n.substring(className.length + 1) : n)
           .toList());
@@ skipping 10 matching lines @@
         .classes[#GeneratedMessage];
     do {
       fillConflictingNames(cls);
       cls = cls.superclass;
     } while (cls != null);
 
     var x = new Set<String>()..addAll(MessageGenerator.reservedNames);
     expect(names.toList()..sort(), equals(x.toList()..sort()));
   });
 }