Add stack chain support to pub.

sdk/lib/_internal/pub/lib/src/source/git.dart

Index: sdk/lib/_internal/pub/lib/src/source/git.dart
diff --git a/sdk/lib/_internal/pub/lib/src/source/git.dart b/sdk/lib/_internal/pub/lib/src/source/git.dart
index 5a7f5e5977b2b0cfeab7be96d29a9a4c0b489417..6bdf3809f7cee83564bbca7c052a7d3797ee5447 100644
--- a/sdk/lib/_internal/pub/lib/src/source/git.dart
+++ b/sdk/lib/_internal/pub/lib/src/source/git.dart
@@ -121,7 +121,7 @@ class GitSource extends Source {
   /// future that completes once this is finished and throws an exception if it
   /// fails.
   Future _ensureRepoCache(PackageId id) {
-    return new Future.sync(() {
+    return syncFuture(() {
       var path = _repoCachePath(id);
       if (!entryExists(path)) return _clone(_getUrl(id), path, mirror: true);
       return git.run(["fetch"], workingDir: path).then((result) => null);
@@ -141,7 +141,7 @@ class GitSource extends Source {
   /// the working tree, but instead makes the repository a local mirror of the
   /// remote repository. See the manpage for `git clone` for more information.
   Future _clone(String from, String to, {bool mirror: false}) {
-    return new Future.sync(() {
+    return syncFuture(() {
       // Git on Windows does not seem to automatically create the destination
       // directory.
       ensureDir(to);