Add support for lazy transformers.

pkg/barback/lib/src/group_runner.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.group_runner;
 
 import 'dart:async';
 
 import 'asset_cascade.dart';
 import 'asset_node.dart';
@@ skipping 42 matching lines @@
       lastPhase = lastPhase.addPhase(phase);
       _phases.add(lastPhase);
     }
 
     for (var phase in _phases) {
       _onDirtyPool.add(phase.onDirty);
       _onLogPool.add(phase.onLog);
     }
   }
 
+  /// Force all [LazyTransformer]s' transforms in this group to begin producing
+  /// concrete assets.
+  void forceAllTransforms() {
+    for (var phase in _phases) {
+      phase.forceAllTransforms();
+    }
+  }
+
   /// Adds a new asset as an input for this group.
   void addInput(AssetNode node) {
     _phases.first.addInput(node);
   }
 
   /// Removes this group and all sub-phases within it.
   void remove() {
     _phases.first.remove();
   }
 
@@ skipping 12 matching lines @@
     var newOutputs = _phases.last.availableOutputs
         .difference(_alreadyEmittedOutputs);
     for (var output in newOutputs) {
       output.whenRemoved(() => _alreadyEmittedOutputs.remove(output));
     }
     _alreadyEmittedOutputs.addAll(newOutputs);
 
     return new Future.value(newOutputs);
   }
 }