Add BaseTransform.consumePrimary to barback.

pkg/barback/lib/src/phase_input.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library barback.phase_input;
 
 import 'dart:async';
 
 import 'asset_forwarder.dart';
 import 'asset_node.dart';
@@ skipping 242 matching lines @@
         });
 
         transform.onAsset.listen((asset) {
           if (asset.id == input.id) {
             _overwritingOutputs.add(asset);
             asset.whenRemoved(_adjustPassThrough);
             _adjustPassThrough();
           }
 
           _onAssetController.add(asset);
-        }, onDone: () => _transforms.remove(transform));
+        }, onDone: () {
+          _transforms.remove(transform);
+          // When a transform is removed, we need to re-adjust the pass-through
+          // in case its call to `consumePrimary` was preventing pass-through
+          _adjustPassThrough();
+        });
 
         _onLogPool.add(transform.onLog);
       });
     }));
   }
 
   /// Adjust whether [input] is passed through the phase unmodified, based on
   /// whether it's overwritten by other transforms in this phase.
   ///
   /// If [input] was already passed-through, this will update the passed-through
   /// value.
   void _adjustPassThrough() {
     // If [input] is removed, [_adjustPassThrough] can still be called due to
     // [TransformNode]s marking their outputs as removed.
     if (!input.state.isAvailable) return;
 
     // If there's an output with the same id as the primary input, that
-    // overwrites the input so it doesn't get passed through. Otherwise,
-    // create a pass-through controller if none exists, or set the existing
-    // one available.
-    if (_overwritingOutputs.isNotEmpty) {
+    // overwrites the input so it doesn't get passed through. A transformer
+    // explicitly consuming the input will also cause it not to get passed
+    // through. Otherwise, create a pass-through controller if none exists, or
+    // set the existing one available.
+    if (_overwritingOutputs.isNotEmpty ||
+        _transforms.any((transform) => transform.consumePrimary)) {
       if (_passThroughController != null) {
         _passThroughController.setRemoved();
         _passThroughController = null;
       }
     } else if (isDirty) {
       // If the input is dirty, we're still figuring out whether a transform
       // will overwrite the input. As such, we shouldn't pass through the asset
       // yet.
     } else if (_passThroughController == null) {
       _passThroughController = new AssetNodeController.from(input);
       _onAssetController.add(_passThroughController.node);
     } else if (_passThroughController.node.state.isDirty) {
       _passThroughController.setAvailable(input.asset);
     }
   }
 
   String toString() => "phase input in $_location for $input";
 }