Fix all strong mode warnings in protoc-plugin

bin/protoc_plugin_bazel.dart

 #!/usr/bin/env dart
 // Copyright (c) 2016, 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:io';
 import 'package:protoc_plugin/bazel.dart';
 import 'package:protoc_plugin/protoc.dart';
 
 void main() {
-  var packages = {};
+  var packages = <String, BazelPackage>{};
   new CodeGenerator(stdin, stdout).generate(
       optionParsers: {bazelOptionId: new BazelOptionParser(packages)},
       config: new BazelOutputConfiguration(packages));
 }