Fix a barback bug that triggered when an asset was removed.

pkg/barback/lib/src/phase.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;
 
 import 'dart:async';
 
 import 'asset.dart';
 import 'asset_cascade.dart';
@@ skipping 123 matching lines @@
 
     _newInputs.clear();
 
     return Future.wait(futures);
   }
 
   /// Applies all currently wired up and dirty transforms.
   ///
   /// Passes their outputs to the next phase.
   Future _processTransforms() {
-    var dirtyTransforms = _transforms.where((transform) => transform.isDirty);
+    // Convert this to a list so we can safely modify _transforms while
+    // iterating over it.
+    var dirtyTransforms = _transforms.where((transform) => transform.isDirty)
+        .toList();
     if (dirtyTransforms.isEmpty) return null;
 
-    return Future.wait(dirtyTransforms.map((transform) => transform.apply()))
-        .then((transformOutputs) {
+    return Future.wait(dirtyTransforms.map((transform) {
+      if (inputs.containsKey(transform.primary.id)) return transform.apply();
+
+      // If the primary input for the transform has been removed, get rid of it
+      // and all its outputs.
+      _transforms.remove(transform);
+      return new Future.value(
+          new TransformOutputs(new AssetSet(), transform.outputs));

[Bob Nystrom, 2013/07/17 17:18:26]

Smart way to handle this. :)

+    })).then((transformOutputs) {
       // Collect all of the outputs. Since the transforms are run in parallel,
       // we have to be careful here to ensure that the result is deterministic
       // and not influenced by the order that transforms complete.
       var updated = new AssetSet();
       var removed = new Set<AssetId>();
       var collisions = new Set<AssetId>();
 
       // Handle the generated outputs of all transforms first.
       for (var outputs in transformOutputs) {
         // Collect the outputs of all transformers together.
@@ skipping 20 matching lines @@
       for (var collision in collisions) {
         cascade.reportError(new AssetCollisionException(collision));
         // TODO(rnystrom): Define what happens after a collision occurs.
       }
 
       // Pass the outputs to the next phase.
       _next.updateInputs(updated, removed);
     });
   }
 }