Fix additional cases where analysis server might send null, violating protocol.

pkg/analysis_server/lib/src/edit/edit_domain.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 edit.domain;
 
 import 'dart:async';
 
 import 'package:analysis_server/edit/assist/assist_core.dart';
 import 'package:analysis_server/edit/fix/fix_core.dart';
@@ skipping 471 matching lines @@
         refactoring = new ExtractLocalRefactoring(units[0], offset, length);
         feedback = new ExtractLocalVariableFeedback([], [], []);
       }
     }
     if (kind == RefactoringKind.EXTRACT_METHOD) {
       List<CompilationUnit> units = server.getResolvedCompilationUnits(file);
       if (units.isNotEmpty) {
         refactoring = new ExtractMethodRefactoring(
             searchEngine, units[0], offset, length);
         feedback = new ExtractMethodFeedback(
-            offset, length, null, [], false, [], [], []);
+            offset, length, '', [], false, [], [], []);
       }
     }
     if (kind == RefactoringKind.INLINE_LOCAL_VARIABLE) {
       List<CompilationUnit> units = server.getResolvedCompilationUnits(file);
       if (units.isNotEmpty) {
         refactoring =
             new InlineLocalRefactoring(searchEngine, units[0], offset);
       }
     }
     if (kind == RefactoringKind.INLINE_METHOD) {
@@ skipping 142 matching lines @@
     }
     if (refactoring is RenameRefactoring) {
       RenameRefactoring renameRefactoring = refactoring;
       RenameOptions renameOptions = params.options;
       renameRefactoring.newName = renameOptions.newName;
       return renameRefactoring.checkNewName();
     }
     return new RefactoringStatus();
   }
 }