Add the ability to dynamically modify transforms to barback.

pkg/barback/lib/src/package_graph.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library barback.package_graph;
 
 import 'dart:async';
 
 import 'asset_cascade.dart';
 import 'asset_id.dart';
@@ skipping 38 matching lines @@
   ///
   /// This will not emit programming errors from barback itself. Those will be
   /// emitted through the [results] stream's error channel.
   Stream<BarbackException> get errors => _errors;
   Stream<BarbackException> _errors;
 
   /// Creates a new [PackageGraph] that will transform assets in all packages
   /// 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();
       _cascades[package] = cascade;
 
       cascade.results.listen((result) {
         _cascadeResults[cascade.package] = result;
         // If any cascade hasn't yet finished, the overall build isn't finished
         // either.
         if (_cascadeResults.values.any((result) => result == null)) return;
@@ skipping 37 matching lines @@
 
   /// Removes [removed] from the graph's known set of source assets.
   void removeSources(Iterable<AssetId> sources) {
     groupBy(sources, (id) => id.package).forEach((package, ids) {
       var cascade = _cascades[package];
       if (cascade == null) throw new ArgumentError("Unknown package $package.");
       _cascadeResults[package] = null;
       cascade.removeSources(ids);
     });
   }
+
+  void updateTransformers(String package,
+      Iterable<Iterable<Transformer>> transformers) {
+    _cascadeResults[package] = null;

[Bob Nystrom, 2013/08/20 19:08:54]

What if the set of transformers are unchanged?

[nweiz, 2013/08/20 21:39:17]

We always want to trigger a rebuild so that the caller can consistently expect
that [updateTransformers] will cause [results] to emit a result at some point in
the future.

[Bob Nystrom, 2013/08/20 22:18:12]

Ensuring a result is sent is fine, but it sucks to discard the previously
generated assets and actually apply all of the transformers again from scratch.
Isn't there a way we can do the former without the latter?

+    _cascades[package].updateTransformers(transformers);
+  }
 }