Avoid O(n^2) behavior in Barback.

pkg/barback/lib/src/group_runner.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.group_runner;
 
 import 'dart:async';
 
 import 'asset_cascade.dart';
 import 'asset_node.dart';
 import 'log.dart';
 import 'phase.dart';
 import 'stream_pool.dart';
 import 'transformer_group.dart';
 
 /// A class that processes all of the phases in a single transformer group.
 ///
 /// A group takes many inputs, processes them, and emits many outputs.
 class GroupRunner {
   /// The group this runner runs.
   final TransformerGroup _group;
 
   /// A string describing the location of [this] in the transformer graph.
   final String _location;
 
   /// The phases defined by this group.
   final _phases = new List<Phase>();
 
   /// Whether [this] is dirty and still has more processing to do.
-  bool get isDirty => _phases.any((phase) => phase.isDirty);
+  bool get isDirty {
+    // Just check the last phase, since it will check all the previous phases
+    // itself.
+    return _phases.last.isDirty;
+  }
 
   /// A stream that emits an event whenever [this] is no longer dirty.
   ///
   /// This is synchronous in order to guarantee that it will emit an event as
   /// soon as [isDirty] flips from `true` to `false`.
   Stream get onDone => _onDoneController.stream;
   final _onDoneController = new StreamController.broadcast(sync: true);
 
   /// A stream that emits any new assets emitted by [this].
   ///
@@ skipping 41 matching lines @@
     _phases.first.addInput(node);
   }
 
   /// Removes this group and all sub-phases within it.
   void remove() {
     _phases.first.remove();
   }
 
   String toString() => "group in phase $_location for $_group";
 }