Fix for renaming import without an import prefix.

pkg/analysis_server/lib/src/services/refactoring/rename.dart

 // Copyright (c) 2014, 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 services.src.refactoring.rename;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/protocol_server.dart' hide Element;
 import 'package:analysis_server/src/services/correction/source_range.dart';
@@ skipping 167 matching lines @@
   bool requiresPreview() {
     return false;
   }
 
   static String _getDisplayName(Element element) {
     if (element is ImportElement) {
       PrefixElement prefix = element.prefix;
       if (prefix != null) {
         return prefix.displayName;
       }
+      return '';
     }
     return element.displayName;
   }
 }