Return 'null' rename refactoring in case of a null element.

pkg/analysis_server/lib/src/services/refactoring/refactoring.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.refactoring;
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart'
     show RefactoringMethodParameter, SourceChange;
@@ skipping 338 matching lines @@
 /**
  * Abstract [Refactoring] for renaming some [Element].
  */
 abstract class RenameRefactoring implements Refactoring {
   /**
    * Returns a new [RenameRefactoring] instance for renaming [element],
    * maybe `null` if there is no support for renaming [Element]s of the given
    * type.
    */
   factory RenameRefactoring(SearchEngine searchEngine, Element element) {
+    if (element == null) {
+      return null;
+    }
     if (element is PropertyAccessorElement) {
       element = (element as PropertyAccessorElement).variable;
     }
     if (element.enclosingElement is CompilationUnitElement) {
       return new RenameUnitMemberRefactoringImpl(searchEngine, element);
     }
     if (element is ConstructorElement) {
       return new RenameConstructorRefactoringImpl(searchEngine, element);
     }
     if (element is ImportElement) {
@@ skipping 34 matching lines @@
    * Validates that the [newName] is a valid identifier and is appropriate for
    * the type of the [Element] being renamed.
    *
    * It does not perform all the checks (such as checking for conflicts with any
    * existing names in any of the scopes containing the current name), as many
    * of these checkes require search engine. Use [checkFinalConditions] for this
    * level of checking.
    */
   RefactoringStatus checkNewName();
 }