Avoid O(n^2) behavior in Barback.

pkg/barback/lib/src/asset_cascade.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.asset_cascade;
 
 import 'dart:async';
 
 import 'asset.dart';
 import 'asset_id.dart';
@@ skipping 46 matching lines @@
   Stream<BarbackException> get errors => _errorsController.stream;
   final _errorsController =
       new StreamController<BarbackException>.broadcast(sync: true);
 
   /// A stream that emits an event whenever any transforms in this cascade logs
   /// an entry.
   Stream<LogEntry> get onLog => _onLogPool.stream;
   final _onLogPool = new StreamPool<LogEntry>.broadcast();
 
   /// 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);
 
   /// Returns all currently-available output assets from this cascade.
   AssetSet get availableOutputs =>
@@ skipping 123 matching lines @@
     _onLogPool.add(phase.onLog);
     phase.onDone.listen((_) {
       if (!isDirty) _onDoneController.add(null);
     });
 
     _phases.add(phase);
   }
 
   String toString() => "cascade for $package";
 }