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

lib/message_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;
 
 const String SP = ' ';
 
-class MessageGenerator implements ProtobufContainer {
+class MessageGenerator extends ProtobufContainer {
   // List of names which cannot be used in a subclass of GeneratedMessage.
   static final List<String> reservedNames =
     ['hashCode', 'noSuchMethod','runtimeType', 'toString',
      'fromBuffer', 'fromJson', 'hasRequiredFields', 'isInitialized',
      'clear', 'getTagNumber', 'check',
      'writeToBuffer', 'writeToCodedBufferWriter',
      'mergeFromCodedBufferReader', 'mergeFromBuffer',
      'writeToJson', 'mergeFromJson',
      'addExtension', 'getExtension', 'setExtension',
      'hasExtension', 'clearExtension',
@@ skipping 220 matching lines @@
             '(${fieldTypeString} v)${SP}'
             '{${SP}setField(${field.number},${SP}v);${SP}}');
         out.println('bool $hasIdentifier()${SP}=>'
             '${SP}hasField(${field.number});');
         out.println('void $clearIdentifier()${SP}=>'
             '${SP}clearField(${field.number});');
       }
     }
   }
 }