Use package names as import prefixes when generating code

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 classname;
 }
 
 class CodeGenerator implements ProtobufContainer {
   final Stream<List<int>> _streamIn;
   final IOSink _streamOut;
   final IOSink _streamErr;
 
   CodeGenerator(this._streamIn, this._streamOut, this._streamErr);
 
   void generate() {
@@ skipping 19 matching lines @@
                 generators.add(generator);
               }
             }
 
             response.file.addAll(
                 generators.map((filegen) => filegen.generateResponse()));
             _streamOut.add(response.writeToBuffer());
         });
   }
 
+  String get package => '';
+  String get classname => null;
   String get fqname => '';
-  String get classname => null;
 }
 
 
 class GenerationOptions {
   final Map<String, String> fieldNameOptions;
 
   GenerationOptions._(this.fieldNameOptions);
 
   // Parse the options in the request. If there was an error in the
   // options null is returned and the error is set on the response.
@@ skipping 37 matching lines @@
     if (errors.length > 0) {
       response.error = errors.join('\n');
       return null;
     } else {
       return new GenerationOptions._(fieldNameOptions);
     }
   }
 
   String fieldNameOption(String fqname) => fieldNameOptions[fqname];
 }