Fix issue with importing packages where the package name included a dot

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;
+  String get packageImportPrefix => package.replaceAll('.', r'$');
 }
 
-class CodeGenerator implements ProtobufContainer {
+class CodeGenerator extends ProtobufContainer {
   final Stream<List<int>> _streamIn;
   final IOSink _streamOut;
   final IOSink _streamErr;
 
   CodeGenerator(this._streamIn, this._streamOut, this._streamErr);
 
   void generate() {
     _streamIn
         .fold(new BytesBuilder(), (builder, data) => builder..add(data))
         .then((builder) => builder.takeBytes())
@@ skipping 76 matching lines @@
     if (errors.length > 0) {
       response.error = errors.join('\n');
       return null;
     } else {
       return new GenerationOptions._(fieldNameOptions);
     }
   }
 
   String fieldNameOption(String fqname) => fieldNameOptions[fqname];
 }