Add support for lazy transformers.

pkg/barback/lib/src/package_graph.dart

Index: pkg/barback/lib/src/package_graph.dart
diff --git a/pkg/barback/lib/src/package_graph.dart b/pkg/barback/lib/src/package_graph.dart
index ce49459f2f886b87a080179f74698e17a532c244..22114f059bb7af83a7006a82b8c61cfbbdf46b18 100644
--- a/pkg/barback/lib/src/package_graph.dart
+++ b/pkg/barback/lib/src/package_graph.dart
@@ -14,7 +14,6 @@ import 'build_result.dart';
 import 'errors.dart';
 import 'log.dart';
 import 'package_provider.dart';
-import 'pool.dart';
 import 'transformer.dart';
 import 'utils.dart';
 
@@ -139,7 +138,14 @@ class PackageGraph {
   /// If a build is currently in progress, waits until it completes. The
   /// returned future will complete with an error if the build is not
   /// successful.
+  ///
+  /// Any transforms using [LazyTransformer]s will be materialized and their
+  /// outputs returned.
   Future<AssetSet> getAllAssets() {
+    for (var cascade in _cascades.values) {
+      cascade.materializeAllTransforms();
+    }
+
     if (_cascadeResults.values.contains(null)) {
       // A build is still ongoing, so wait for it to complete and try again.
       return results.first.then((_) => getAllAssets());