Support passing barback assets through phases in which they're unused.

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 252ee1c9e312fee7d88f9fe191922b3a87fff1f8..445e86f529b37047aad447d5b30883aa20c2b408 100644
--- a/pkg/barback/test/transformer/check_content.dart
+++ b/pkg/barback/test/transformer/check_content.dart
@@ -10,15 +10,15 @@ import 'package:barback/barback.dart';
 
 import 'mock.dart';
 
-/// A transformer that modifies assets with the given content.
+/// A transformer that modifies assets that contains the given content.
 class CheckContentTransformer extends MockTransformer {
-  final String content;
+  final Pattern content;
   final String addition;
 
   CheckContentTransformer(this.content, this.addition);
 
   Future<bool> doIsPrimary(Asset asset) =>
-    asset.readAsString().then((value) => value == content);
+    asset.readAsString().then((value) => value.contains(content));
 
   Future doApply(Transform transform) {
     return getPrimary(transform).then((primary) {