Fix a subtle bug in barback.

pkg/barback/lib/src/phase.dart

Index: pkg/barback/lib/src/phase.dart
diff --git a/pkg/barback/lib/src/phase.dart b/pkg/barback/lib/src/phase.dart
index 48fd46f748dc60a865358f0df8487a9e529e0028..1230bea094e36970d402547ec6b386c868c33ec0 100644
--- a/pkg/barback/lib/src/phase.dart
+++ b/pkg/barback/lib/src/phase.dart
@@ -148,6 +148,7 @@ class Phase {
       // 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;
@@ -188,8 +189,9 @@ class Phase {
             _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);
           }
         }