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

pkg/analysis_server/lib/src/services/refactoring/extract_method.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.extract_method;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/protocol_server.dart' hide Element;
 import 'package:analysis_server/src/services/correction/name_suggestion.dart';
@@ skipping 52 matching lines @@
   final CompilationUnit unit;
   final int selectionOffset;
   final int selectionLength;
   AnalysisContext context;
   CompilationUnitElement unitElement;
   LibraryElement libraryElement;
   SourceRange selectionRange;
   CorrectionUtils utils;
   Set<LibraryElement> librariesToImport = new Set<LibraryElement>();
 
-  String returnType;
+  String returnType = '';
   String variableType;
   String name;
   bool extractAll = true;
   bool canCreateGetter = false;
   bool createGetter = false;
   final List<String> names = <String>[];
   final List<int> offsets = <int>[];
   final List<int> lengths = <int>[];
 
   /**
@@ skipping 1117 matching lines @@
       return false;
     }
     for (int i = 0; i < parameterTypes.length; i++) {
       if (other.parameterTypes[i] != parameterTypes[i]) {
         return false;
       }
     }
     return true;
   }
 }