Don't make lazy transformers eager when an asset is requested.

pkg/barback/lib/src/asset_node.dart

Index: pkg/barback/lib/src/asset_node.dart
diff --git a/pkg/barback/lib/src/asset_node.dart b/pkg/barback/lib/src/asset_node.dart
index 36a3a669801a37d58068088136fb571ad3abe5e1..16aff4064f4aabd006e910c6d5718128f1346f71 100644
--- a/pkg/barback/lib/src/asset_node.dart
+++ b/pkg/barback/lib/src/asset_node.dart
@@ -58,9 +58,10 @@ class AssetNode {
   /// lazy.
   Function _lazyCallback;
 
-  /// Whether this is lazy and needs [force] to be called before it will be
-  /// marked available.
-  bool get isLazy => _lazyCallback != null;
+  /// Whether this asset's transform is deferred.
+  ///
+  /// See [TransformNode.deferred].
+  bool get deferred => transform != null && transform.deferred;
 
   /// A broadcast stream that emits an event whenever the node changes state.
   ///
@@ -195,9 +196,14 @@ class AssetNodeController {
   /// Marks the node as [AssetState.DIRTY].
   void setDirty() {
     assert(node._state != AssetState.REMOVED);
-    node._state = AssetState.DIRTY;
     node._asset = null;
     node._lazyCallback = null;
+
+    // Don't re-emit a dirty event to avoid cases where we try to dispatch an
+    // event while handling another event (e.g. an output is marked lazy, which
+    // causes it to be forced, which causes it to be marked dirty).
+    if (node._state.isDirty) return;
+    node._state = AssetState.DIRTY;
     node._stateChangeController.add(AssetState.DIRTY);
   }