Minor clean-ups and rename

pkg/analysis_server/lib/src/plugin/server_plugin.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.
 
 library analysis_server.src.plugin.server_plugin;
 
 import 'package:analysis_server/plugin/plugin.dart';
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/domain_analysis.dart';
 import 'package:analysis_server/src/domain_completion.dart';
@@ skipping 83 matching lines @@
   }
 
   /**
    * Validate the given extension by throwing an [ExtensionError] if it is not a
    * valid domain.
    */
   void _validateDomainExtension(Object extension) {
     if (extension is! RequestHandlerFactory) {
       String id = domainExtensionPoint.uniqueIdentifier;
       throw new ExtensionError(
-          'Extensions to the $id must be a RequestHandlerFactory');
+          'Extensions to $id must be a RequestHandlerFactory');
     }
   }
 }