Run pub tests against older versions of barback.

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

Index: third_party/pkg/barback-0.13.0/test/transformer/create_asset.dart
diff --git a/pkg/barback/test/transformer/create_asset.dart b/third_party/pkg/barback-0.13.0/test/transformer/create_asset.dart
similarity index 63%
copy from pkg/barback/test/transformer/create_asset.dart
copy to third_party/pkg/barback-0.13.0/test/transformer/create_asset.dart
index 82e80b37d4ba8a3e681ae5066126aa5482800c6d..5db8940fd7226c45615858c098edaf557084e1aa 100644
--- a/pkg/barback/test/transformer/create_asset.dart
+++ b/third_party/pkg/barback-0.13.0/test/transformer/create_asset.dart
@@ -4,7 +4,10 @@
 
 library barback.test.transformer.create_asset;
 
+import 'dart:async';
+
 import 'package:barback/barback.dart';
+import 'package:barback/src/utils.dart';
 
 import 'mock.dart';
 
@@ -14,10 +17,12 @@ class CreateAssetTransformer extends MockTransformer {
 
   CreateAssetTransformer(this.output);
 
-  bool doIsPrimary(AssetId id) => true;
+  Future<bool> doIsPrimary(_) => new Future.value(true);
 
-  void doApply(Transform transform) {
-    transform.addOutput(
-        new Asset.fromString(new AssetId.parse(output), output));
+  Future doApply(Transform transform) {
+    return newFuture(() {
+      transform.addOutput(
+          new Asset.fromString(new AssetId.parse(output), output));
+    });
   }
 }