Make barback more package-aware.

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 65dcad6cf3491a34ecaff1314542ab97669bb1f1..9ebca8e608ab57273230922ffd5d3fe3334d64d3 100644
--- a/pkg/barback/lib/src/errors.dart
+++ b/pkg/barback/lib/src/errors.dart
@@ -36,3 +36,14 @@ class MissingInputException implements Exception {
 
   String toString() => "Missing input $id.";
 }
+
+/// Error thrown when a transformer outputs an asset with the wrong package
+/// name.
+class InvalidOutputException implements Exception {
+  final String package;
+  final AssetId id;
+
+  InvalidOutputException(this.package, this.id);
+
+  String toString() => "Invalid output $id: must be in package $package.";
+}