Move barback to a more event-based model.

pkg/barback/lib/src/transform.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.transform;
 
 import 'dart:async';
 
 import 'asset.dart';
 import 'asset_id.dart';
@@ skipping 49 matching lines @@
     return newFuture(() {
       var node = _node.phase.inputs[id];
       // TODO(rnystrom): Need to handle passthrough where an asset from a
       // previous phase can be found.
 
       // Throw if the input isn't found. This ensures the transformer's apply
       // is exited. We'll then catch this and report it through the proper
       // results stream.
       if (node == null) throw new MissingInputException(id);
 
-      // Keep track of which assets this transform depends on.
-      _inputs.add(node);
-      return node.asset;
+      // If the asset node is found, wait until its contents are actually
+      // available before we return them.
+      return node.whenAvailable.then((asset) {
+        _inputs.add(node);
+        return asset;
+      }).catchError((error) {
+        if (error is! AssetNotFoundException || error.id != id) throw error;
+        // If the node was removed before it could be loaded, treat it as though
+        // it never existed and throw a MissingInputException.
+        throw new MissingInputException(id);
+      });
     });
   }
 
   /// Stores [output] as the output created by this transformation.
   ///
   /// A transformation can output as many assets as it wants.
   void addOutput(Asset output) {
     // TODO(rnystrom): This should immediately throw if an output with that ID
     // has already been created by this transformer.
     _outputs.add(output);
   }
 }