Make listDir and createSymlink synchronous in pub.

utils/pub/validator/license.dart

Index: utils/pub/validator/license.dart
diff --git a/utils/pub/validator/license.dart b/utils/pub/validator/license.dart
index 6cd623b643fd569ac89ee62e5a6f39fcc7da87eb..ffcbd10c9deb8b2e669348bfa659aa31ac2547f9 100644
--- a/utils/pub/validator/license.dart
+++ b/utils/pub/validator/license.dart
@@ -19,10 +19,14 @@ class LicenseValidator extends Validator {
     : super(entrypoint);
 
   Future validate() {
-    return listDir(entrypoint.root.dir).then((files) {
+    return new Future.of(() {
       var licenseLike = new RegExp(
           r"^([a-zA-Z0-9]+[-_])?(LICENSE|COPYING)(\..*)?$");
-      if (files.map(path.basename).any(licenseLike.hasMatch)) return;
+      if (listDir(entrypoint.root.dir)
+          .map(path.basename)
+          .any(licenseLike.hasMatch)) {
+        return;
+      }
 
       errors.add(
           "You must have a COPYING or LICENSE file in the root directory.\n"