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

lib/extension_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;
 
-class ExtensionGenerator implements ProtobufContainer {
+class ExtensionGenerator extends ProtobufContainer {
   final String fqname;
   final FieldDescriptorProto _descriptor;
   final ProtobufContainer _parent;
   final GenerationContext _context;
 
   ExtensionGenerator(
       FieldDescriptorProto descriptor,
       ProtobufContainer parent,
       GenerationContext context)
           : this._descriptor = descriptor,
@@ skipping 61 matching lines @@
 
     out.println('static final Extension $name${SP}=${SP}'
       'new Extension(\'$extendee\',${SP}\'$name\',${SP}'
       '${_descriptor.number},${SP}GeneratedMessage.$type'
       '${initializer}${builder}${valueOf}'
       ');');
   }
 
   String get name => classname;
 }