Fix async/await type checking in analyzer.

pkg/analysis_server/lib/src/edit/edit_domain.dart

Index: pkg/analysis_server/lib/src/edit/edit_domain.dart
diff --git a/pkg/analysis_server/lib/src/edit/edit_domain.dart b/pkg/analysis_server/lib/src/edit/edit_domain.dart
index 5e2723cf33e8ab1fe768f740eba8a8d4212083ef..fd9b24f756bca9bded0ec94a9842e4bdd3788d05 100644
--- a/pkg/analysis_server/lib/src/edit/edit_domain.dart
+++ b/pkg/analysis_server/lib/src/edit/edit_domain.dart
@@ -398,7 +398,7 @@ class _RefactoringManager {
         this.file == file &&
         this.offset == offset &&
         this.length == length) {
-      return null;
+      return;
     }
     _reset();
     this.kind = kind;
@@ -495,7 +495,7 @@ class _RefactoringManager {
     if (refactoring == null) {
       initStatus =
           new RefactoringStatus.fatal('Unable to create a refactoring');
-      return null;
+      return;
     }
     // check initial conditions
     initStatus = await refactoring.checkInitialConditions();
@@ -539,7 +539,6 @@ class _RefactoringManager {
       feedback.elementKindName = refactoring.elementKindName;
       feedback.oldName = refactoring.oldName;
     }
-    return null;
   }
 
   void _reset([engine.AnalysisContext context]) {