Avoid O(n^2) behavior in Barback.

pkg/barback/lib/src/asset_cascade.dart

Index: pkg/barback/lib/src/asset_cascade.dart
diff --git a/pkg/barback/lib/src/asset_cascade.dart b/pkg/barback/lib/src/asset_cascade.dart
index 633184afbbf584359a38a57ea15bc862b782e334..b87ca39ae874cb26c74e72928d13b87ffcd8ba28 100644
--- a/pkg/barback/lib/src/asset_cascade.dart
+++ b/pkg/barback/lib/src/asset_cascade.dart
@@ -64,7 +64,11 @@ class AssetCascade {
   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.
   ///