Add BaseTransform.consumePrimary to barback.

pkg/barback/lib/src/transform_node.dart

Index: pkg/barback/lib/src/transform_node.dart
diff --git a/pkg/barback/lib/src/transform_node.dart b/pkg/barback/lib/src/transform_node.dart
index bc899c68c407ca9692fef33de4c23aec1a106c65..152ef1623c5ed18c9aaf9505b95edd389338d3f3 100644
--- a/pkg/barback/lib/src/transform_node.dart
+++ b/pkg/barback/lib/src/transform_node.dart
@@ -55,6 +55,13 @@ class TransformNode {
   /// Whether [_apply] is currently running.
   var _isApplying = false;
 
+  /// Whether the most recent run of this transform has declared that it
+  /// consumes the primary input.
+  ///
+  /// Defaults to `false`.
+  bool get consumePrimary => _consumePrimary;
+  bool _consumePrimary = false;
+
   /// Whether [transformer] is lazy and this transform has yet to be forced.
   bool _isLazy;
 
@@ -246,6 +253,8 @@ class TransformNode {
     }).then((_) {
       if (_hasBecomeDirty || _onAssetController.isClosed) return;
 
+      _consumePrimary = transformController.consumePrimary;
+
       var newOutputs = transformController.outputs;
       // Any ids that are for a different package are invalid.
       var invalidIds = newOutputs
@@ -289,6 +298,8 @@ class TransformNode {
     }).then((_) {
       if (_hasBecomeDirty || _onAssetController.isClosed) return;
 
+      _consumePrimary = transformController.consumePrimary;
+
       var newIds = transformController.outputIds;
       var invalidIds =
           newIds.where((id) => id.package != phase.cascade.package).toSet();