Preserve laziness in declaring transformers.

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 'asset_cascade.dart';
 import 'asset_id.dart';
@@ skipping 152 matching lines @@
   /// will automatically begin determining which transforms can consume it as a
   /// primary input. The transforms themselves won't be applied until [process]
   /// is called, however.
   ///
   /// This should only be used for brand-new assets or assets that have been
   /// removed and re-created. The phase will automatically handle updated assets
   /// using the [AssetNode.onStateChange] stream.
   void addInput(AssetNode node) {
     if (_inputs.containsKey(node.id)) _inputs[node.id].remove();
 
-    node.force();
-
     // Each group is one channel along which an asset may be forwarded, as is
     // each transformer.
     var forwarder = new PhaseForwarder(
         node, _transformers.length, _groups.length);
     _forwarders[node.id] = forwarder;
     forwarder.onAsset.listen(_handleOutputWithoutForwarder);
     if (forwarder.output != null) {
       _handleOutputWithoutForwarder(forwarder.output);
     }
 
@@ skipping 193 matching lines @@
     assert(asset.state.isDirty);
     asset.force();
     asset.whenStateChanges().then((state) {
       if (state.isRemoved) return getOutput(asset.id);
       return asset;
     }).then(request.complete).catchError(request.completeError);
   }
 
   String toString() => "phase $_location.$_index";
 }