Fix a subtle bug in barback.

pkg/barback/lib/src/phase_output.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_output;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'asset_cascade.dart';
@@ skipping 34 matching lines @@
 
   PhaseOutput(this._phase, AssetNode output)
       : _outputController = new AssetNodeController.from(output) {
     assert(!output.state.isRemoved);
     add(output);
   }
 
   /// Adds an asset node as an output with this id.
   void add(AssetNode node) {
     assert(node.id == output.id);
+    assert(!output.state.isRemoved);
     _assets.add(node);
     _watchAsset(node);
   }
 
   /// Removes all existing listeners on [output] without actually closing
   /// [this].
   ///
   /// This marks [output] as removed, but immediately replaces it with a new
   /// [AssetNode] in the same state as the old output. This is used when adding
   /// a new [Phase] to cause consumers of the prior phase's outputs to be to
@@ skipping 52 matching lines @@
     // If there's still a collision, report it. This lets the user know
     // if they've successfully resolved the collision or not.
     if (_assets.length > 1) {
       // Pump the event queue to ensure that the removal of the input triggers
       // a new build to which we can attach the error.
       // TODO(nweiz): report this through the output asset.
       newFuture(() => _phase.cascade.reportError(collisionException));
     }
   }
 }