Fix a subtle bug in barback.

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 130 matching lines @@
     _next = new Phase(cascade, transformers);
     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;
+      output.output.whenRemoved.then((_) => _outputs.remove(output.output.id));
       _next.addInput(output.output);
     }
     return _next;
   }
 
   /// Mark this phase as removed.
   ///
   /// This will remove all the phase's outputs and all following phases.
   void remove() {
     removeFollowing();
@@ skipping 20 matching lines @@
     var outputIds = new Set<AssetId>();
     return Future.wait(_inputs.values.map((input) {
       if (!input.isDirty) return new Future.value(new Set());
       return input.process().then((outputs) {
         for (var asset in outputs) {
           outputIds.add(asset.id);
           if (_outputs.containsKey(asset.id)) {
             _outputs[asset.id].add(asset);
           } else {
             _outputs[asset.id] = new PhaseOutput(this, asset);
-            _outputs[asset.id].output.whenRemoved
-                .then((_) => _outputs.remove(asset.id));
+            _outputs[asset.id].output.whenRemoved.then((_) {
+              _outputs.remove(asset.id);
+            });
             if (_next != null) _next.addInput(_outputs[asset.id].output);
           }
         }
       });
     })).then((_) {
       // Report collisions in a deterministic order.
       outputIds = outputIds.toList();
       outputIds.sort((a, b) => a.compareTo(b));
       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);
       }
     });
   }
 }