Support transformers that depend on other transformers.

pkg/barback/lib/src/utils.dart

Index: pkg/barback/lib/src/utils.dart
diff --git a/pkg/barback/lib/src/utils.dart b/pkg/barback/lib/src/utils.dart
index 37816bcfe8120bee4732c7b8070f303ccc8dff87..95f498e037a634f3c0aac681663b242b57be6882 100644
--- a/pkg/barback/lib/src/utils.dart
+++ b/pkg/barback/lib/src/utils.dart
@@ -83,7 +83,7 @@ String byteToHex(int byte) {
 ///
 /// This returns a map whose keys are the return values of [fn] and whose values
 /// are lists of each element in [iter] for which [fn] returned that key.
-Map groupBy(Iterable iter, fn(element)) {
+Map<Object, List> groupBy(Iterable iter, fn(element)) {
   var map = {};
   for (var element in iter) {
     var list = map.putIfAbsent(fn(element), () => []);