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 49374679a43bee895d28f98bcc41489b88b7efa9..7e8a15c9b3ffd018e832bda844b58fe7409a34c8 100644
--- a/pkg/barback/lib/src/phase_input.dart
+++ b/pkg/barback/lib/src/phase_input.dart
@@ -150,6 +150,14 @@ class PhaseInput {
     if (_adjustTransformersFuture == null) _adjustTransformers();
   }
 
+  /// Force all [LazyTransformer]s' transforms in this input to begin producing
+  /// concrete assets.
+  void forceAllTransforms() {
+    for (var transform in _transforms) {
+      transform.force();
+    }
+  }
+
   /// Asynchronously determines which transformers can consume [input] as a
   /// primary input and creates transforms for them.
   ///