Move barback to a more event-based model.

pkg/barback/lib/src/package_provider.dart

Index: pkg/barback/lib/src/package_provider.dart
diff --git a/pkg/barback/lib/src/package_provider.dart b/pkg/barback/lib/src/package_provider.dart
index b2a3495fd58691af8498dc5aa22db1318196a348..031caf870e2b7cee0222f7cfc5f6424f3ed4498a 100644
--- a/pkg/barback/lib/src/package_provider.dart
+++ b/pkg/barback/lib/src/package_provider.dart
@@ -27,5 +27,9 @@ abstract class PackageProvider {
   /// the next. All [Transformer]s in a single phase will be run in parallel.
   Iterable<Iterable<Transformer>> getTransformers(String package);
 
+  /// Loads an asset from disk.
+  ///
+  /// This should be re-entrant; it may be called multiple times with the same
+  /// id before the previously returned future has completed.
   Future<Asset> getAsset(AssetId id);
 }