Support loading transformer plugins from pub.

pkg/barback/lib/src/phase_input.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_input;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'asset.dart';
@@ skipping 118 matching lines @@
         _passThroughController == null) {
       _passThroughController =
           new AssetNodeController.available(input.asset, input.transform);
       _newPassThrough = true;
     }
 
     var brandNewTransformers = newTransformers.difference(oldTransformers);
     if (brandNewTransformers.isEmpty) return;
 
     brandNewTransformers.forEach(_transformers.add);
-    _adjustTransformers();
+    if (_adjustTransformersFuture == null) _adjustTransformers();
   }
 
   /// Asynchronously determines which transformers can consume [input] as a
   /// primary input and creates transforms for them.
   ///
   /// This ensures that if [input] is modified or removed during or after the
   /// time it takes to adjust its transformers, they're appropriately
   /// re-adjusted. Its progress can be tracked in [_adjustTransformersFuture].
   void _adjustTransformers() {
     // Mark the input as dirty. This may not actually end up creating any new
@@ skipping 138 matching lines @@
       return new Future.value(
           new Set<AssetNode>.from([_passThroughController.node]));
     }
 
     return Future.wait(_transforms.map((transform) {
       if (!transform.isDirty) return new Future.value(new Set());
       return transform.apply();
     })).then((outputs) => unionAll(outputs));
   }
 }