Add stack chain support to pub.

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

Index: sdk/lib/_internal/pub/lib/src/validator/lib.dart
diff --git a/sdk/lib/_internal/pub/lib/src/validator/lib.dart b/sdk/lib/_internal/pub/lib/src/validator/lib.dart
index 98bed28b94f9e82a07e820e6df9a504d55ebf3e9..22ed406e2e99800739da8c2d09ce38fbad304f9a 100644
--- a/sdk/lib/_internal/pub/lib/src/validator/lib.dart
+++ b/sdk/lib/_internal/pub/lib/src/validator/lib.dart
@@ -10,6 +10,7 @@ import 'package:path/path.dart' as path;
 
 import '../entrypoint.dart';
 import '../io.dart';
+import '../utils.dart';
 import '../validator.dart';
 
 // TODO(nweiz): When issue 7196 is fixed, complain about non-Dart files in lib.
@@ -20,7 +21,7 @@ class LibValidator extends Validator {
     : super(entrypoint);
 
   Future validate() {
-    return new Future.sync(() {
+    return syncFuture(() {
       var libDir = path.join(entrypoint.root.dir, "lib");
 
       if (!dirExists(libDir)) {