Fix all strong mode warnings in protoc-plugin

test/enum_generator_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 enum_generator_test;
 
 import 'package:protoc_plugin/indenting_writer.dart';
 import 'package:protoc_plugin/protoc.dart';
 import 'package:protoc_plugin/src/descriptor.pb.dart';
 import 'package:test/test.dart';
 
 void main() {
   test('testEnumGenerator', () {
     String expected = r'''
 class PhoneType extends ProtobufEnum {
   static const PhoneType MOBILE = const PhoneType._(0, 'MOBILE');
   static const PhoneType HOME = const PhoneType._(1, 'HOME');
   static const PhoneType WORK = const PhoneType._(2, 'WORK');
 
   static const PhoneType BUSINESS = WORK;
 
   static const List<PhoneType> values = const <PhoneType> [
     MOBILE,
     HOME,
     WORK,
   ];
 
-  static final Map<int, PhoneType> _byValue = ProtobufEnum.initByValue(values);
-  static PhoneType valueOf(int value) => _byValue[value];
+  static final Map<int, dynamic> _byValue = ProtobufEnum.initByValue(values);
+  static PhoneType valueOf(int value) => _byValue[value] as PhoneType;
   static void $checkItem(PhoneType v) {
     if (v is !PhoneType) checkItemFailed(v, 'PhoneType');
   }
 
   const PhoneType._(int v, String n) : super(v, n);
 }
 
 ''';
     EnumDescriptorProto ed = new EnumDescriptorProto()
-        ..name = 'PhoneType'
-        ..value.addAll([
-            new EnumValueDescriptorProto()
-                ..name = 'MOBILE'
-                ..number = 0,
-            new EnumValueDescriptorProto()
-                ..name = 'HOME'
-                ..number = 1,
-            new EnumValueDescriptorProto()
-                ..name = 'WORK'
-                ..number = 2,
-            new EnumValueDescriptorProto()
-                ..name = 'BUSINESS'
-                ..number = 2]);
+      ..name = 'PhoneType'
+      ..value.addAll([
+        new EnumValueDescriptorProto()
+          ..name = 'MOBILE'
+          ..number = 0,
+        new EnumValueDescriptorProto()
+          ..name = 'HOME'
+          ..number = 1,
+        new EnumValueDescriptorProto()
+          ..name = 'WORK'
+          ..number = 2,
+        new EnumValueDescriptorProto()
+          ..name = 'BUSINESS'
+          ..number = 2
+      ]);
     IndentingWriter writer = new IndentingWriter();
     EnumGenerator eg = new EnumGenerator(ed, null);
     eg.generate(writer);
     expect(writer.toString(), expected);
   });
 }