Rename mixin protos and add a check for undefined mixin name

lib/src/plugin.pb.dart

Index: lib/src/plugin.pb.dart
diff --git a/lib/src/plugin.pb.dart b/lib/src/plugin.pb.dart
index 8a6a34dba51059c4c550ab318eed3de6e87690fc..ff4e2f90b136a0e4338afbb3b98e5b61445cfb41 100644
--- a/lib/src/plugin.pb.dart
+++ b/lib/src/plugin.pb.dart
@@ -4,6 +4,7 @@
 library proto2.compiler_plugin;
 
 import 'package:protobuf/protobuf.dart';
+
 import 'descriptor.pb.dart' as proto2;
 
 class CodeGeneratorRequest extends GeneratedMessage {
@@ -116,30 +117,3 @@ class CodeGeneratorResponse extends GeneratedMessage {
 
 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},
-  ],
-};
-