Add support for lazy transformers.

pkg/barback/lib/src/asset_cascade.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.asset_cascade;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'asset.dart';
@@ skipping 133 matching lines @@
     // * [id] may be generated before the compilation is finished. We should
     //   be able to quickly check whether there are any more in-place
     //   transformations that can be run on it. If not, we can return it early.
     // * If [id] has never been generated and all active transformers provide
     //   metadata about the file names of assets it can emit, we can prove that
     //   none of them can emit [id] and fail early.
     return _phases.last.getOutput(id).then((node) {
       // If the requested asset is available, we can just return it.
       if (node != null && node.state.isAvailable) return node;
 
-      // If there's a build running, that build might generate the asset, so we
-      // wait for it to complete and then try again.
       if (_processDone != null) {
+        // If there's a build running, that build might generate the asset, so
+        // we wait for it to complete and then try again.
         return _processDone.then((_) => getAssetNode(id));
       }
 
       // If the asset hasn't been built and nothing is building now, the asset
       // won't be generated, so we return null.
       return null;
     });
   }
 
   /// Adds [sources] to the graph's known set of source assets.
@@ skipping 59 matching lines @@
     }
 
     if (transformers.length == 0) {
       _phases.last.updateTransformers([]);
     } else if (transformers.length < _phases.length) {
       _phases[transformers.length - 1].removeFollowing();
       _phases.removeRange(transformers.length, _phases.length);
     }
   }
 
+  /// Force all [LazyTransformer]s' transforms in this cascade to begin
+  /// producing concrete assets.
+  void forceAllTransforms() {
+    for (var phase in _phases) {
+      phase.forceAllTransforms();
+    }
+  }
+
   void reportError(BarbackException error) {
     _accumulatedErrors.add(error);
     _errorsController.add(error);
   }
 
   /// Add [phase] to the end of [_phases] and watch its [onDirty] stream.
   void _addPhase(Phase phase) {
     _onDirtyPool.add(phase.onDirty);
     _onLogPool.add(phase.onLog);
     phase.onDirty.listen((_) {
@@ skipping 56 matching lines @@
 
         // Otherwise, everything is done.
         return null;
       }
 
       // Process that phase and then loop onto the next.
       return future.then((_) => _process());
     });
   }
 }