Make listDir and createSymlink synchronous in pub.

utils/pub/path_source.dart

Index: utils/pub/path_source.dart
diff --git a/utils/pub/path_source.dart b/utils/pub/path_source.dart
index 3b433a1c46fdca8f25416dc9c1dd4b8aa4eecb53..1d2b339d09afa889ea655b968ec9886ec605a4fe 100644
--- a/utils/pub/path_source.dart
+++ b/utils/pub/path_source.dart
@@ -47,9 +47,10 @@ class PathSource extends Source {
         return false;
       }
 
-      return createPackageSymlink(id.name, id.description["path"], destination,
+      createPackageSymlink(id.name, id.description["path"], destination,
           relative: id.description["relative"]);
-    }).then((_) => true);
+      return true;
+    });
   }
 
   /// Parses a path dependency. This takes in a path string and returns a map.