Add the ability to dynamically modify transforms to barback.

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 639deea6f49300cb47f6e4729e07d787a45a93e6..85c250b38219402403dbe18228752859a90a2f89 100644
--- a/pkg/barback/lib/src/package_graph.dart
+++ b/pkg/barback/lib/src/package_graph.dart
@@ -56,8 +56,7 @@ class PackageGraph {
   /// made available by [provider].
   PackageGraph(this.provider) {
     for (var package in provider.packages) {
-      var cascade = new AssetCascade(this, package,
-          provider.getTransformers(package));
+      var cascade = new AssetCascade(this, package);
       // The initial result for each cascade is "success" since the cascade
       // doesn't start building until some source in that graph is updated.
       _cascadeResults[package] = new BuildResult.success();
@@ -115,4 +114,10 @@ class PackageGraph {
       cascade.removeSources(ids);
     });
   }
+
+  void updateTransformers(String package,
+      Iterable<Iterable<Transformer>> transformers) {
+    _cascadeResults[package] = null;
+    _cascades[package].updateTransformers(transformers);
+  }
 }