Support passing barback assets through phases in which they're unused.

pkg/barback/lib/src/errors.dart

Index: pkg/barback/lib/src/errors.dart
diff --git a/pkg/barback/lib/src/errors.dart b/pkg/barback/lib/src/errors.dart
index a379ae516f41217a23475e2ee8123c7ffb0f7aa6..f3560664ed6982c5a3923de62b2a134887c29b3a 100644
--- a/pkg/barback/lib/src/errors.dart
+++ b/pkg/barback/lib/src/errors.dart
@@ -29,6 +29,10 @@ abstract class BarbackException implements Exception {}
 /// Error thrown when two or more transformers both output an asset with [id].
 class AssetCollisionException implements BarbackException {
   /// All the transforms that output an asset with [id].
+  ///
+  /// If this only contains a single transform, that indicates that a
+  /// transformer produced an output that collides with a source asset or an
+  /// asset from a previous phase.
   final Set<TransformInfo> transforms;
   final AssetId id;