Fix all strong mode warnings in protoc-plugin

lib/src/plugin.pb.dart

 ///
 //  Generated code. Do not modify.
 ///
 library proto2.compiler_plugin;
 
 import 'package:protobuf/protobuf.dart';
 import 'descriptor.pb.dart' as proto2;
 
 class CodeGeneratorRequest extends GeneratedMessage {
   static final BuilderInfo _i = new BuilderInfo('CodeGeneratorRequest')
-    ..p(1, 'fileToGenerate', PbFieldType.PS)
-    ..a(2, 'parameter', PbFieldType.OS)
-    ..pp(15, 'protoFile', PbFieldType.PM, proto2.FileDescriptorProto.$checkItem, proto2.FileDescriptorProto.create)
+    ..p/*<String>*/(1, 'fileToGenerate', PbFieldType.PS)
+    ..a/*<String>*/(2, 'parameter', PbFieldType.OS)
+    ..pp/*<proto2.FileDescriptorProto>*/(15, 'protoFile', PbFieldType.PM, proto2.FileDescriptorProto.$checkItem, proto2.FileDescriptorProto.create)
   ;
 
   CodeGeneratorRequest() : super();
   CodeGeneratorRequest.fromBuffer(List<int> i, [ExtensionRegistry r = ExtensionRegistry.EMPTY]) : super.fromBuffer(i, r);
   CodeGeneratorRequest.fromJson(String i, [ExtensionRegistry r = ExtensionRegistry.EMPTY]) : super.fromJson(i, r);
   CodeGeneratorRequest clone() => new CodeGeneratorRequest()..mergeFromMessage(this);
   BuilderInfo get info_ => _i;
   static CodeGeneratorRequest create() => new CodeGeneratorRequest();
   static PbList<CodeGeneratorRequest> createRepeated() => new PbList<CodeGeneratorRequest>();
   static CodeGeneratorRequest getDefault() {
     if (_defaultInstance == null) _defaultInstance = new _ReadonlyCodeGeneratorRequest();
     return _defaultInstance;
   }
   static CodeGeneratorRequest _defaultInstance;
   static void $checkItem(CodeGeneratorRequest v) {
     if (v is !CodeGeneratorRequest) checkItemFailed(v, 'CodeGeneratorRequest');
   }
 
-  List<String> get fileToGenerate => getField(1);
+  List<String> get fileToGenerate => $_get(0, 1, null);
 
-  String get parameter => getField(2);
-  void set parameter(String v) { setField(2, v); }
-  bool hasParameter() => hasField(2);
+  String get parameter => $_get(1, 2, '');
+  void set parameter(String v) { $_setString(1, 2, v); }
+  bool hasParameter() => $_has(1, 2);
   void clearParameter() => clearField(2);
 
-  List<proto2.FileDescriptorProto> get protoFile => getField(15);
+  List<proto2.FileDescriptorProto> get protoFile => $_get(2, 15, null);
 }
 
 class _ReadonlyCodeGeneratorRequest extends CodeGeneratorRequest with ReadonlyMessageMixin {}
 
 class CodeGeneratorResponse_File extends GeneratedMessage {
   static final BuilderInfo _i = new BuilderInfo('CodeGeneratorResponse_File')
-    ..a(1, 'name', PbFieldType.OS)
-    ..a(2, 'insertionPoint', PbFieldType.OS)
-    ..a(15, 'content', PbFieldType.OS)
+    ..a/*<String>*/(1, 'name', PbFieldType.OS)
+    ..a/*<String>*/(2, 'insertionPoint', PbFieldType.OS)
+    ..a/*<String>*/(15, 'content', PbFieldType.OS)
     ..hasRequiredFields = false
   ;
 
   CodeGeneratorResponse_File() : super();
   CodeGeneratorResponse_File.fromBuffer(List<int> i, [ExtensionRegistry r = ExtensionRegistry.EMPTY]) : super.fromBuffer(i, r);
   CodeGeneratorResponse_File.fromJson(String i, [ExtensionRegistry r = ExtensionRegistry.EMPTY]) : super.fromJson(i, r);
   CodeGeneratorResponse_File clone() => new CodeGeneratorResponse_File()..mergeFromMessage(this);
   BuilderInfo get info_ => _i;
   static CodeGeneratorResponse_File create() => new CodeGeneratorResponse_File();
   static PbList<CodeGeneratorResponse_File> createRepeated() => new PbList<CodeGeneratorResponse_File>();
   static CodeGeneratorResponse_File getDefault() {
     if (_defaultInstance == null) _defaultInstance = new _ReadonlyCodeGeneratorResponse_File();
     return _defaultInstance;
   }
   static CodeGeneratorResponse_File _defaultInstance;
   static void $checkItem(CodeGeneratorResponse_File v) {
     if (v is !CodeGeneratorResponse_File) checkItemFailed(v, 'CodeGeneratorResponse_File');
   }
 
-  String get name => getField(1);
-  void set name(String v) { setField(1, v); }
-  bool hasName() => hasField(1);
+  String get name => $_get(0, 1, '');
+  void set name(String v) { $_setString(0, 1, v); }
+  bool hasName() => $_has(0, 1);
   void clearName() => clearField(1);
 
-  String get insertionPoint => getField(2);
-  void set insertionPoint(String v) { setField(2, v); }
-  bool hasInsertionPoint() => hasField(2);
+  String get insertionPoint => $_get(1, 2, '');
+  void set insertionPoint(String v) { $_setString(1, 2, v); }
+  bool hasInsertionPoint() => $_has(1, 2);
   void clearInsertionPoint() => clearField(2);
 
-  String get content => getField(15);
-  void set content(String v) { setField(15, v); }
-  bool hasContent() => hasField(15);
+  String get content => $_get(2, 15, '');
+  void set content(String v) { $_setString(2, 15, v); }
+  bool hasContent() => $_has(2, 15);
   void clearContent() => clearField(15);
 }
 
 class _ReadonlyCodeGeneratorResponse_File extends CodeGeneratorResponse_File with ReadonlyMessageMixin {}
 
 class CodeGeneratorResponse extends GeneratedMessage {
   static final BuilderInfo _i = new BuilderInfo('CodeGeneratorResponse')
-    ..a(1, 'error', PbFieldType.OS)
-    ..pp(15, 'file', PbFieldType.PM, CodeGeneratorResponse_File.$checkItem, CodeGeneratorResponse_File.create)
+    ..a/*<String>*/(1, 'error', PbFieldType.OS)
+    ..pp/*<CodeGeneratorResponse_File>*/(15, 'file', PbFieldType.PM, CodeGeneratorResponse_File.$checkItem, CodeGeneratorResponse_File.create)
     ..hasRequiredFields = false
   ;
 
   CodeGeneratorResponse() : super();
   CodeGeneratorResponse.fromBuffer(List<int> i, [ExtensionRegistry r = ExtensionRegistry.EMPTY]) : super.fromBuffer(i, r);
   CodeGeneratorResponse.fromJson(String i, [ExtensionRegistry r = ExtensionRegistry.EMPTY]) : super.fromJson(i, r);
   CodeGeneratorResponse clone() => new CodeGeneratorResponse()..mergeFromMessage(this);
   BuilderInfo get info_ => _i;
   static CodeGeneratorResponse create() => new CodeGeneratorResponse();
   static PbList<CodeGeneratorResponse> createRepeated() => new PbList<CodeGeneratorResponse>();
   static CodeGeneratorResponse getDefault() {
     if (_defaultInstance == null) _defaultInstance = new _ReadonlyCodeGeneratorResponse();
     return _defaultInstance;
   }
   static CodeGeneratorResponse _defaultInstance;
   static void $checkItem(CodeGeneratorResponse v) {
     if (v is !CodeGeneratorResponse) checkItemFailed(v, 'CodeGeneratorResponse');
   }
 
-  String get error => getField(1);
-  void set error(String v) { setField(1, v); }
-  bool hasError() => hasField(1);
+  String get error => $_get(0, 1, '');
+  void set error(String v) { $_setString(0, 1, v); }
+  bool hasError() => $_has(0, 1);
   void clearError() => clearField(1);
 
-  List<CodeGeneratorResponse_File> get file => getField(15);
+  List<CodeGeneratorResponse_File> get file => $_get(1, 15, null);
 }
 
 class _ReadonlyCodeGeneratorResponse extends CodeGeneratorResponse with ReadonlyMessageMixin {}
 
 const CodeGeneratorRequest$json = const {
   '1': 'CodeGeneratorRequest',
   '2': const [
     const {'1': 'file_to_generate', '3': 1, '4': 3, '5': 9},
     const {'1': 'parameter', '3': 2, '4': 1, '5': 9},
     const {'1': 'proto_file', '3': 15, '4': 3, '5': 11, '6': '.proto2.FileDescriptorProto'},
@@ skipping 11 matching lines @@
 
 const CodeGeneratorResponse_File$json = const {
   '1': 'File',
   '2': const [
     const {'1': 'name', '3': 1, '4': 1, '5': 9},
     const {'1': 'insertion_point', '3': 2, '4': 1, '5': 9},
     const {'1': 'content', '3': 15, '4': 1, '5': 9},
   ],
 };