Add stack chain support to pub.

sdk/lib/_internal/pub/lib/src/validator/directory.dart

Index: sdk/lib/_internal/pub/lib/src/validator/directory.dart
diff --git a/sdk/lib/_internal/pub/lib/src/validator/directory.dart b/sdk/lib/_internal/pub/lib/src/validator/directory.dart
index 6039fc5c3b02f9be04a4568fe87e1bfab218d64c..f733e4e42d71f995c7d092c1d9fae947e2dc78e5 100644
--- a/sdk/lib/_internal/pub/lib/src/validator/directory.dart
+++ b/sdk/lib/_internal/pub/lib/src/validator/directory.dart
@@ -10,6 +10,7 @@ import 'package:path/path.dart' as path;
 
 import '../entrypoint.dart';
 import '../io.dart';
+import '../utils.dart';
 import '../validator.dart';
 
 /// A validator that validates a package's top-level directories.
@@ -22,7 +23,7 @@ class DirectoryValidator extends Validator {
   ];
 
   Future validate() {
-    return new Future.sync(() {
+    return syncFuture(() {
       for (var dir in listDir(entrypoint.root.dir)) {
         if (!dirExists(dir)) continue;