First pass at build dependency graph for barback.

pkg/barback/lib/barback.dart

Index: pkg/barback/lib/barback.dart
diff --git a/tests/lib/async/run_async_test.dart b/pkg/barback/lib/barback.dart
similarity index 60%
copy from tests/lib/async/run_async_test.dart
copy to pkg/barback/lib/barback.dart
index 0ec7afae1fa6b2c289bc09a3c4fe067f382c0896..da8dd3773fb1fb1290c4de4aa2b395d71566842f 100644
--- a/tests/lib/async/run_async_test.dart
+++ b/pkg/barback/lib/barback.dart
@@ -2,13 +2,12 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-library run_async_test;
+library barback;
 
 import 'dart:async';
-import '../../../pkg/unittest/lib/unittest.dart';
+import 'dart:io';
 
-main() {
-  test("run async test", () {
-    runAsync(expectAsync0(() {}));
-  });
-}
+export 'src/asset.dart';
+export 'src/asset_id.dart';
+export 'src/asset_provider.dart';
+export 'src/errors.dart';