Add stack chain support to pub.

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

Index: sdk/lib/_internal/pub/lib/src/source/path.dart
diff --git a/sdk/lib/_internal/pub/lib/src/source/path.dart b/sdk/lib/_internal/pub/lib/src/source/path.dart
index 76892e1e6528a26573858f4073dff48b9382476a..27f129722408936a50f11a2ae065ba3ab335b1ea 100644
--- a/sdk/lib/_internal/pub/lib/src/source/path.dart
+++ b/sdk/lib/_internal/pub/lib/src/source/path.dart
@@ -20,7 +20,7 @@ class PathSource extends Source {
   final shouldCache = false;
 
   Future<Pubspec> describeUncached(PackageId id) {
-    return new Future.sync(() {
+    return syncFuture(() {
       var dir = _validatePath(id.name, id.description);
       return new Pubspec.load(dir, systemCache.sources,
           expectedName: id.name);
@@ -35,7 +35,7 @@ class PathSource extends Source {
   }
 
   Future<bool> get(PackageId id, String destination) {
-    return new Future.sync(() {
+    return syncFuture(() {
       try {
         var dir = _validatePath(id.name, id.description);
         createPackageSymlink(id.name, dir, destination,