Enable cross-package transforms to see new transformers in other packages.

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 'dart:collection';
 
 import 'asset_cascade.dart';
@@ skipping 121 matching lines @@
       input.updateTransformers(_transformers);
     }
   }
 
   /// Add a new phase after this one with [transformers].
   ///
   /// This may only be called on a phase with no phase following it.
   Phase addPhase(Iterable<Transformer> transformers) {
     assert(_next == null);
     _next = new Phase(cascade, transformers);
-    for (var outputs in _outputs.values) {
-      _next.addInput(outputs.output);
+    for (var output in _outputs.values.toList()) {
+      // Remove [output]'s listeners because now they should get the asset from
+      // [_next], rather than this phase. Any transforms consuming [output] will
+      // be re-run and will consume the output from the new final phase.
+      output.removeListeners();
+
+      // Removing [output]'s listeners will cause it to be removed from
+      // [_outputs], so we have to put it back.
+      _outputs[output.output.id] = output;
+      _next.addInput(output.output);
     }
     return _next;
   }
 
   /// Processes this phase.
   ///
   /// Returns a future that completes when processing is done. If there is
   /// nothing to process, returns `null`.
   Future process() {
     if (!_inputs.values.any((input) => input.isDirty)) return null;
@@ skipping 21 matching lines @@
       for (var id in outputIds) {
         // It's possible the output was removed before other transforms in this
         // phase finished.
         if (!_outputs.containsKey(id)) continue;
         var exception = _outputs[id].collisionException;
         if (exception != null) cascade.reportError(exception);
       }
     });
   }
 }