Rename mixin protos and add a check for undefined mixin name

lib/code_generator.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.
 
 part of protoc;
 
 abstract class ProtobufContainer {
   String get package;
   String get classname;
   String get fqname;
@@ skipping 45 matching lines @@
       List<FileGenerator> generators = <FileGenerator>[];
       for (FileDescriptorProto file in request.protoFile) {
         generators.add(new FileGenerator(file));
       }
 
       // Collect field types and importable files.
       link(options, generators);
 
       // Generate the .pb.dart file if requested.
       for (var gen in generators) {
-        var name = gen._fileDescriptor.name;
+        var name = gen.descriptor.name;
         if (request.fileToGenerate.contains(name)) {
           response.file.addAll(gen.generateFiles(config));
         }
       }
       _streamOut.add(response.writeToBuffer());
     });
   }
 
   String get package => '';
   String get classname => null;
   String get fqname => '';
   get fileGen => null;
 }