Make Phase.getInput in barback play nicely with the push model.

pkg/barback/lib/src/transform_node.dart

Index: pkg/barback/lib/src/transform_node.dart
diff --git a/pkg/barback/lib/src/transform_node.dart b/pkg/barback/lib/src/transform_node.dart
index 2fa826e2f9e10030b9f980fbeff53a8d07e83b6d..8aeb657ee922ba9d8e9a3dcdccefff89fb6a8274 100644
--- a/pkg/barback/lib/src/transform_node.dart
+++ b/pkg/barback/lib/src/transform_node.dart
@@ -229,19 +229,10 @@ class TransformNode {
       // results stream.
       if (node == null) throw new MissingInputException(info, id);
 
-      // If the asset node is found, wait until its contents are actually
-      // available before we return them.
-      return node.whenAvailable((asset) {
-        _inputSubscriptions.putIfAbsent(node.id,
-            () => node.onStateChange.listen((_) => _dirty()));
-
-        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(info, id);
-      });
+      _inputSubscriptions.putIfAbsent(node.id,
+          () => node.onStateChange.listen((_) => _dirty()));
+
+      return node.asset;
     });
   }