Rename mixin protos and add a check for undefined mixin name

lib/linker.dart

 // Copyright (c) 2015, 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;
 
 /// Resolves all cross-references in a set of proto files.
 void link(GenerationOptions options, Iterable<FileGenerator> files) {
   GenerationContext ctx = new GenerationContext(options);
 
@@ skipping 30 matching lines @@
 
   /// The types available to proto fields.
   final Map<String, ProtobufContainer> _typeRegistry =
       <String, ProtobufContainer>{};
 
   GenerationContext(this.options);
 
   /// Makes info about a .pb.dart file available for reference,
   /// using the filename given to us by protoc.
   void registerProtoFile(FileGenerator f) {
-    _files[f._fileDescriptor.name] = f;
+    _files[f.descriptor.name] = f;
   }
 
   /// Makes a message, group, or enum available for reference.
   void registerFieldType(String name, ProtobufContainer type) {
     _typeRegistry[name] = type;
   }
 
   /// Returns info about a .pb.dart being imported,
   /// based on the filename given to us by protoc.
   FileGenerator getImportedProtoFile(String name) => _files[name];
 
   /// Returns info about the type of a message, group, or enum field,
   /// based on the fully qualified name given to us by protoc.
   ProtobufContainer getFieldType(String name) => _typeRegistry[name];
 }