Don't treat the primary input as a secondary input in barback.

pkg/barback/lib/src/transform.dart

Index: pkg/barback/lib/src/transform.dart
diff --git a/pkg/barback/lib/src/transform.dart b/pkg/barback/lib/src/transform.dart
index 25c260af53a809be0af5c067fb6af2f9d3de7a0a..268849d78a419d2faa567df217ffc3560e948854 100644
--- a/pkg/barback/lib/src/transform.dart
+++ b/pkg/barback/lib/src/transform.dart
@@ -57,7 +57,10 @@ class Transform extends BaseTransform {
   /// Gets the asset for an input [id].
   ///
   /// If an input with [id] cannot be found, throws an [AssetNotFoundException].
-  Future<Asset> getInput(AssetId id) => _node.getInput(id);
+  Future<Asset> getInput(AssetId id) {
+    if (id == _node.primary.id) return syncFuture(() => primaryInput);
+    return _node.getInput(id);
+  }
 
   /// A convenience method to the contents of the input with [id] as a string.
   ///