Re-run a transform when a secondary input starts existing.

pkg/barback/lib/src/group_runner.dart

Index: pkg/barback/lib/src/group_runner.dart
diff --git a/pkg/barback/lib/src/group_runner.dart b/pkg/barback/lib/src/group_runner.dart
index aefda2c7fc67414e8b81a6087d1b93de42f229bd..970d4aa4ceed9599176883c6b2ea0c6a2e6be6e7 100644
--- a/pkg/barback/lib/src/group_runner.dart
+++ b/pkg/barback/lib/src/group_runner.dart
@@ -53,8 +53,8 @@ class GroupRunner {
   final _onLogPool = new StreamPool<LogEntry>.broadcast();
 
   GroupRunner(AssetCascade cascade, this._group, this._location) {
-    _addPhase(new Phase(cascade, _location), _group.phases.first);
-    for (var phase in _group.phases.skip(1)) {
+    _addPhase(new Phase(cascade, _location), []);
+    for (var phase in _group.phases) {
       _addPhase(_phases.last.addPhase(), phase);
     }