Add the ability to dynamically modify transforms to barback.

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 54 matching lines @@
 
 /// 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]));
 
+/// 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.
 Stream mergeStreams(Iterable<Stream> streams) {
   streams = streams.toList();
   var doneCount = 0;
   // Use a sync stream to preserve the synchrony behavior of the input streams.
   // If the inputs are sync, then this will be sync as well; if the inputs are
   // async, then the events we receive will also be async, and forwarding them
   // sync won't change that.
   var controller = new StreamController(sync: true);
 
@@ skipping 33 matching lines @@
   // We use a delayed future to allow runAsync events to finish. The
   // Future.value or Future() constructors use runAsync themselves and would
   // therefore not wait for runAsync callbacks that are scheduled after invoking
   // this method.
   return new Future.delayed(Duration.ZERO, () => pumpEventQueue(times - 1));
 }
 
 /// Like [new Future], but avoids issue 11911 by using [new Future.value] under
 /// the covers.
 Future newFuture(callback()) => new Future.value().then((_) => callback());