Add support for lazy transformers.

pkg/barback/lib/src/phase.dart

Index: pkg/barback/lib/src/phase.dart
diff --git a/pkg/barback/lib/src/phase.dart b/pkg/barback/lib/src/phase.dart
index 2136a1281e94d65e04d1914441a4b94ec7faeb90..9aa5484bea828a301df3d0c349ff9e500932176d 100644
--- a/pkg/barback/lib/src/phase.dart
+++ b/pkg/barback/lib/src/phase.dart
@@ -136,6 +136,8 @@ class Phase {
   void addInput(AssetNode node) {
     if (_inputs.containsKey(node.id)) _inputs[node.id].remove();
 
+    node.materialize();

[Bob Nystrom, 2014/01/30 19:33:44]

Why is this being materialized?

[nweiz, 2014/01/31 03:43:27]

The phase needs its inputs to be available if it's going to process them. A
future CL will make it so that lazy assets are piped through dry run
transformers correctly, which will make this materialization happen closer to
where it's specifically needed.

+
     // Each group is one channel along which an asset may be forwarded. Then
     // there's one additional channel for the non-grouped transformers.
     var forwarder = new PhaseForwarder(_groups.length + 1);
@@ -182,7 +184,7 @@ class Phase {
     return newFuture(() {
       if (id.package != cascade.package) return cascade.graph.getAssetNode(id);
       if (!_outputs.containsKey(id)) return null;
-      return _outputs[id].output;
+      return _outputs[id].output..materialize();

[Bob Nystrom, 2014/01/30 19:33:44]

Nit, but I'm really not a fan of using a single cascade just to get back to the
original receiver.

[nweiz, 2014/01/31 03:43:27]

I think it's nicer than making a variable, but okay.

     });
   }
 
@@ -219,6 +221,17 @@ class Phase {
     }
   }
 
+  /// Materialize all [LazyTransformer]s' transforms in this group.
+  void materializeAllTransforms() {
+    for (var group in _groups.values) {
+      group.materializeAllTransforms();
+    }
+
+    for (var input in _inputs.values) {
+      input.materializeAllTransforms();
+    }
+  }
+
   /// Add a new phase after this one with [transformers].
   ///
   /// This may only be called on a phase with no phase following it.