Get rid of unneeded Future.immediate() calls.

utils/pub/git_source.dart

Index: utils/pub/git_source.dart
diff --git a/utils/pub/git_source.dart b/utils/pub/git_source.dart
index 0a3e2b9721f09b459e551eb147192759f2719dea..477cc17a7e9bd17ed250476516aed5e259e423f4 100644
--- a/utils/pub/git_source.dart
+++ b/utils/pub/git_source.dart
@@ -49,11 +49,11 @@ class GitSource extends Source {
       revisionCachePath = path;
       return exists(revisionCachePath);
     }).then((exists) {
-      if (exists) return new Future.immediate(null);
+      if (exists) return;
       return _clone(_repoCachePath(id), revisionCachePath, mirror: false);
     }).then((_) {
       var ref = _getEffectiveRef(id);
-      if (ref == 'HEAD') return new Future.immediate(null);
+      if (ref == 'HEAD') return;
       return _checkOut(revisionCachePath, ref);
     }).then((_) {
       return Package.load(id.name, revisionCachePath, systemCache.sources);