Run pub tests against older versions of barback.

third_party/pkg/barback-0.13.0/test/transformer/emit_nothing.dart

Index: third_party/pkg/barback-0.13.0/test/transformer/emit_nothing.dart
diff --git a/pkg/barback/test/transformer/emit_nothing.dart b/third_party/pkg/barback-0.13.0/test/transformer/emit_nothing.dart
similarity index 75%
copy from pkg/barback/test/transformer/emit_nothing.dart
copy to third_party/pkg/barback-0.13.0/test/transformer/emit_nothing.dart
index 9cf513446ea228c15ef205fb32abd188d2749dfa..a414cb891733c5a6dfafd040e5d5ff35f3f816dd 100644
--- a/pkg/barback/test/transformer/emit_nothing.dart
+++ b/third_party/pkg/barback-0.13.0/test/transformer/emit_nothing.dart
@@ -4,6 +4,8 @@
 
 library barback.test.transformer.emit_nothing;
 
+import 'dart:async';
+
 import 'package:barback/barback.dart';
 
 import 'mock.dart';
@@ -14,11 +16,10 @@ class EmitNothingTransformer extends MockTransformer {
 
   EmitNothingTransformer(this.extension);
 
-  bool doIsPrimary(AssetId id) => id.extension == ".$extension";
+  Future<bool> doIsPrimary(AssetId id) =>
+    new Future.value(id.extension == ".$extension");
 
-  void doApply(Transform transform) {
-    // Emit nothing.
-  }
+  Future doApply(Transform transform) => new Future.value();
 
   String toString() => "$extension->nothing";
 }