Add support for lazy 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';
 import 'dart:typed_data';
 
 import 'package:stack_trace/stack_trace.dart';
@@ skipping 122 matching lines @@
     }
   }
   helper(nested);
   return result;
 }
 
 /// Returns the union of all elements in each set in [sets].
 Set unionAll(Iterable<Set> sets) =>
   sets.fold(new Set(), (union, set) => union.union(set));
 
-/// Passes each key/value pair in [map] to [fn] and returns a new [Map] whose
-/// values are the return values of [fn].
-Map mapMapValues(Map map, fn(key, value)) =>
-  new Map.fromIterable(map.keys, value: (key) => fn(key, map[key]));
+/// Creates a new map from [map] with new keys and values.
+///
+/// The return values of [keyFn] are used as the keys and the return values of
+/// [valueFn] are used as the values for the new map.
+Map mapMap(Map map, keyFn(key, value), valueFn(key, value)) =>
+  new Map.fromIterable(map.keys,
+      key: (key) => keyFn(key, map[key]),
+      value: (key) => valueFn(key, map[key]));
+
+/// Creates a new map from [map] with the same keys.
+///
+/// The return values of [fn] are used as the values for the new map.
+Map mapMapValues(Map map, fn(key, value)) => mapMap(map, (key, _) => key, fn);
+
+/// Creates a new map from [map] with the same keys.
+///
+/// The return values of [fn] are used as the keys for the new map.
+Map mapMapKeys(Map map, fn(key, value)) => mapMap(map, fn, (_, value) => value);
 
 /// Returns whether [set1] has exactly the same elements as [set2].
 bool setEquals(Set set1, Set set2) =>
   set1.length == set2.length && set1.containsAll(set2);
 
 /// Merges [streams] into a single stream that emits events from all sources.
 ///
 /// If [broadcast] is true, this will return a broadcast stream; otherwise, it
 /// will return a buffered stream.
 Stream mergeStreams(Iterable<Stream> streams, {bool broadcast: false}) {
@@ skipping 111 matching lines @@
     subscription = callback().listen(controller.add,
         onError: controller.addError,
         onDone: controller.close);
   },
       onCancel: () => subscription.cancel(),
       onPause: () => subscription.pause(),
       onResume: () => subscription.resume(),
       sync: true);
   return controller.stream;
 }