Fix async/await type checking in analyzer.

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/src/analysis_server.dart';
 import 'package:analysis_server/src/collections.dart';
@@ skipping 380 matching lines @@
    * parameters. The existing [Refactoring] is reused or created as needed.
    */
   Future _init(RefactoringKind kind, String file,
       int offset, int length) async {
     await server.onAnalysisComplete;
     // check if we can continue with the existing Refactoring instance
     if (this.kind == kind &&
         this.file == file &&
         this.offset == offset &&
         this.length == length) {
-      return null;
+      return;
     }
     _reset();
     this.kind = kind;
     this.file = file;
     this.offset = offset;
     this.length = length;
     // simulate an exception
     if (test_simulateRefactoringException_init) {
       throw 'A simulated refactoring exception - init.';
     }
@@ skipping 76 matching lines @@
         }
         // do create the refactoring
         refactoring = new RenameRefactoring(searchEngine, element);
         feedback =
             new RenameFeedback(node.offset, node.length, 'kind', 'oldName');
       }
     }
     if (refactoring == null) {
       initStatus =
           new RefactoringStatus.fatal('Unable to create a refactoring');
-      return null;
+      return;
     }
     // check initial conditions
     initStatus = await refactoring.checkInitialConditions();
     if (refactoring is ExtractLocalRefactoring) {
       ExtractLocalRefactoring refactoring = this.refactoring;
       ExtractLocalVariableFeedback feedback = this.feedback;
       feedback.names = refactoring.names;
       feedback.offsets = refactoring.offsets;
       feedback.lengths = refactoring.lengths;
     }
@@ skipping 23 matching lines @@
             refactoring.isDeclaration,
             className: refactoring.className);
       }
     }
     if (refactoring is RenameRefactoring) {
       RenameRefactoring refactoring = this.refactoring;
       RenameFeedback feedback = this.feedback;
       feedback.elementKindName = refactoring.elementKindName;
       feedback.oldName = refactoring.oldName;
     }
-    return null;
   }
 
   void _reset([engine.AnalysisContext context]) {
     kind = null;
     offset = null;
     length = null;
     refactoring = null;
     feedback = null;
     initStatus = new RefactoringStatus();
     optionsStatus = new RefactoringStatus();
@@ skipping 53 matching lines @@
     }
     if (refactoring is RenameRefactoring) {
       RenameRefactoring renameRefactoring = refactoring;
       RenameOptions renameOptions = params.options;
       renameRefactoring.newName = renameOptions.newName;
       return renameRefactoring.checkNewName();
     }
     return new RefactoringStatus();
   }
 }