Don't pass an asset through a transformer that produces an error.

pkg/barback/lib/src/transform_node.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.transform_node;
 
 import 'dart:async';
 
 import 'asset.dart';
 import 'asset_id.dart';
@@ skipping 110 matching lines @@
   ///
   /// This causes all of the transform's outputs to be marked as removed as
   /// well. Normally this will be automatically done internally based on events
   /// from the primary input, but it's possible for a transform to no longer be
   /// valid even if its primary input still exists.
   void remove() {
     _onAssetController.close();
     _onDoneController.close();
     _primarySubscription.cancel();
     _clearInputSubscriptions();
-    for (var controller in _outputControllers.values) {
-      controller.setRemoved();
-    }
+    _clearOutputs();
     if (_passThroughController != null) {
       _passThroughController.setRemoved();
       _passThroughController = null;
     }
   }
 
   /// If [transformer] is lazy, ensures that its concrete outputs will be
   /// generated.
   void force() {
     // TODO(nweiz): we might want to have a timeout after which, if the
@@ skipping 53 matching lines @@
       phase.cascade.reportError(error);
 
       return false;
     }).then((isPrimary) {
       if (_isRemoved) return;
       if (_state.needsIsPrimary) {
         _process();
       } else if (isPrimary) {
         _apply();
       } else {
-        _doesNotApply();
+        _clearOutputs();
+        _emitPassThrough();
         _state = _TransformNodeState.NOT_PRIMARY;
         _onDoneController.add(null);
       }
     });
   }
 
   /// Applies this transform.
   void _apply() {
     assert(!_onAssetController.isClosed);
 
@@ skipping 15 matching lines @@
       if (!_state.isProcessing || _isRemoved) return;
 
       if (error is! MissingInputException) {
         error = new TransformerException(info, error, stackTrace);
       }
 
       // Catch all transformer errors and pipe them to the results stream. This
       // is so a broken transformer doesn't take down the whole graph.
       phase.cascade.reportError(error);
 
-      _doesNotApply();
+      _clearOutputs();
+      _dontEmitPassThrough();
     }).then((_) {
       if (_isRemoved) return;
 
       if (_state.needsIsPrimary) {
         _process();
       } else if (_state.needsApply) {
         _apply();
       } else {
         assert(_state.isProcessing);
         _state = _TransformNodeState.APPLIED;
@@ skipping 124 matching lines @@
   }
 
   /// Cancels all subscriptions to secondary input nodes.
   void _clearInputSubscriptions() {
     for (var subscription in _inputSubscriptions.values) {
       subscription.cancel();
     }
     _inputSubscriptions.clear();
   }
 
-  /// Marks this transformer as not applying to [primary].
-  ///
-  /// This might be because [primary] isn't primary for [transformer], or
-  /// because [transformer] threw an error during [transformer.apply].
-  void _doesNotApply() {
+  /// Removes all output assets.
+  void _clearOutputs() {
     // Remove all the previously-emitted assets.
     for (var controller in _outputControllers.values) {
       controller.setRemoved();
     }
     _outputControllers.clear();
-    _emitPassThrough();
   }
 
   /// Emit the pass-through asset if it's not being emitted already.
   void _emitPassThrough() {
     assert(!_outputControllers.containsKey(primary.id));
 
     if (_consumePrimary) return;
     if (_passThroughController == null) {
       _passThroughController = new AssetNodeController.from(primary);
       _onAssetController.add(_passThroughController.node);
@@ skipping 74 matching lines @@
   ///
   /// Specifically, whether [this] is [APPLIED] or [NOT_PRIMARY].
   bool get isDone => isApplied || isNotPrimary;
 
   final String name;
 
   const _TransformNodeState._(this.name);
 
   String toString() => name;
 }