Only run [declareOutputs] once for each asset/transformer pair.

pkg/barback/test/transformer/check_content.dart

Index: pkg/barback/test/transformer/check_content.dart
diff --git a/pkg/barback/test/transformer/check_content.dart b/pkg/barback/test/transformer/check_content.dart
index 445e86f529b37047aad447d5b30883aa20c2b408..0696c72c9480aaac12082de39862e85c773f8e25 100644
--- a/pkg/barback/test/transformer/check_content.dart
+++ b/pkg/barback/test/transformer/check_content.dart
@@ -17,12 +17,13 @@ class CheckContentTransformer extends MockTransformer {
 
   CheckContentTransformer(this.content, this.addition);
 
-  Future<bool> doIsPrimary(Asset asset) =>
-    asset.readAsString().then((value) => value.contains(content));
+  Future<bool> doIsPrimary(AssetId id) => new Future.value(true);
 
   Future doApply(Transform transform) {
     return getPrimary(transform).then((primary) {
       return primary.readAsString().then((value) {
+        if (!value.contains(content)) return;
+
         transform.addOutput(
             new Asset.fromString(primary.id, "$value$addition"));
       });