Add support for lazy transformers.

pkg/barback/lib/src/phase_input.dart

Index: pkg/barback/lib/src/phase_input.dart
diff --git a/pkg/barback/lib/src/phase_input.dart b/pkg/barback/lib/src/phase_input.dart
index 13bc700c4a6a906daa3958c07852c829adfb8eed..33312b830a3067d8a2bf12170646cc909055021a 100644
--- a/pkg/barback/lib/src/phase_input.dart
+++ b/pkg/barback/lib/src/phase_input.dart
@@ -146,6 +146,13 @@ class PhaseInput {
     if (_adjustTransformersFuture == null) _adjustTransformers();
   }
 
+  /// Materialize all [LazyTransformer]s' transforms for this input.
+  void materializeAllTransforms() {
+    for (var transform in _transforms) {
+      transform.materialize();
+    }
+  }
+
   /// Asynchronously determines which transformers can consume [input] as a
   /// primary input and creates transforms for them.
   ///