Rename mixin protos and add a check for undefined mixin name

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/*<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();
@@ skipping 92 matching lines @@
   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 => $_get(1, 15, null);
 }
 
 class _ReadonlyCodeGeneratorResponse extends CodeGeneratorResponse with ReadonlyMessageMixin {}
 
-const CodeGeneratorRequest$json = const {
-  '1': 'CodeGeneratorRequest',
-  '2': const [
-    const {'1': 'file_to_generate', '3': 1, '4': 3, '5': 9},
-    const {'1': 'parameter', '3': 2, '4': 1, '5': 9},
-    const {'1': 'proto_file', '3': 15, '4': 3, '5': 11, '6': '.proto2.FileDescriptorProto'},
-  ],
-};
-
-const CodeGeneratorResponse$json = const {
-  '1': 'CodeGeneratorResponse',
-  '2': const [
-    const {'1': 'error', '3': 1, '4': 1, '5': 9},
-    const {'1': 'file', '3': 15, '4': 3, '5': 11, '6': '.proto2.compiler.CodeGeneratorResponse.File'},
-  ],
-  '3': const [CodeGeneratorResponse_File$json],
-};
-
-const CodeGeneratorResponse_File$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},
-  ],
-};
-