Support transformers that depend on other transformers.

pkg/barback/lib/src/utils.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.utils;
 
 import 'dart:async';
 
 /// A pair of values.
 class Pair<E, F> {
@@ skipping 65 matching lines @@
   assert(byte >= 0 && byte <= 255);
 
   const DIGITS = "0123456789abcdef";
   return DIGITS[(byte ~/ 16) % 16] + DIGITS[byte % 16];
 }
 
 /// Group the elements in [iter] by the value returned by [fn].
 ///
 /// 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), () => []);
     list.add(element);
   }
   return map;
 }
 
 /// Flattens nested lists inside an iterable into a single list containing only
 /// non-list elements.
@@ skipping 92 matching lines @@
     stream.listen(
         controller.add,
         onError: (error) => controller.addError(error),
         onDone: controller.close);
   }).catchError((e) {
     controller.addError(e);
     controller.close();
   });
   return controller.stream;
 }