Add stack chain support to pub.

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

Index: sdk/lib/_internal/pub/lib/src/validator/license.dart
diff --git a/sdk/lib/_internal/pub/lib/src/validator/license.dart b/sdk/lib/_internal/pub/lib/src/validator/license.dart
index 222d693206b817dd16a5da28487f91690ad86bb2..ffad6a7d9181f854b47fa78cf35fda1edbc6d763 100644
--- a/sdk/lib/_internal/pub/lib/src/validator/license.dart
+++ b/sdk/lib/_internal/pub/lib/src/validator/license.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 checks that a LICENSE-like file exists.
@@ -18,7 +19,7 @@ class LicenseValidator extends Validator {
     : super(entrypoint);
 
   Future validate() {
-    return new Future.sync(() {
+    return syncFuture(() {
       var licenseLike = new RegExp(
           r"^([a-zA-Z0-9]+[-_])?(LICENSE|COPYING)(\..*)?$");
       if (listDir(entrypoint.root.dir)