Use async/await in refactorings.

pkg/analysis_server/lib/src/services/refactoring/refactoring_internal.dart

Index: pkg/analysis_server/lib/src/services/refactoring/refactoring_internal.dart
diff --git a/pkg/analysis_server/lib/src/services/refactoring/refactoring_internal.dart b/pkg/analysis_server/lib/src/services/refactoring/refactoring_internal.dart
index 0654b95c7bfb7f102b2f78c46e3583a34d0fed8d..ebb09d32bd225c00c2d7071380377fb46a424ef5 100644
--- a/pkg/analysis_server/lib/src/services/refactoring/refactoring_internal.dart
+++ b/pkg/analysis_server/lib/src/services/refactoring/refactoring_internal.dart
@@ -45,18 +45,14 @@ abstract class RefactoringImpl implements Refactoring {
   final List<String> potentialEditIds = <String>[];
 
   @override
-  Future<RefactoringStatus> checkAllConditions() {
+  Future<RefactoringStatus> checkAllConditions() async {
     RefactoringStatus result = new RefactoringStatus();
-    return checkInitialConditions().then((status) {
-      result.addStatus(status);
-      if (result.hasFatalError) {
-        return result;
-      }
-      return checkFinalConditions().then((status) {
-        result.addStatus(status);
-        return result;
-      });
-    });
+    result.addStatus(await checkInitialConditions());
+    if (result.hasFatalError) {
+      return result;
+    }
+    result.addStatus(await checkFinalConditions());
+    return result;
   }
 }