Make listDir and createSymlink synchronous in pub.

utils/tests/pub/descriptor/tar.dart

Index: utils/tests/pub/descriptor/tar.dart
diff --git a/utils/tests/pub/descriptor/tar.dart b/utils/tests/pub/descriptor/tar.dart
index 18af2b38fdbf70351aa9704e4bd6ab7fc32c8d4e..88b0dc150d34741021232acb0ce44decbd2b9ac2 100644
--- a/utils/tests/pub/descriptor/tar.dart
+++ b/utils/tests/pub/descriptor/tar.dart
@@ -27,8 +27,9 @@ class TarFileDescriptor extends DirectoryDescriptor {
       return Future.wait(contents.map((entry) {
         return entry.create(tempDir);
       })).then((_) {
-        return listDir(tempDir, recursive: true, includeHiddenFiles: true);
-      }).then((createdContents) {
+        var createdContents = listDir(tempDir,
+            recursive: true,
+            includeHiddenFiles: true);
         return createTarGz(createdContents, baseDir: tempDir).toBytes();
       }).then((bytes) {
         var file = path.join(parent, name);