Fix a bug when loading secondary assets in barback.

pkg/barback/lib/src/graph/group_runner.dart

Index: pkg/barback/lib/src/graph/group_runner.dart
diff --git a/pkg/barback/lib/src/graph/group_runner.dart b/pkg/barback/lib/src/graph/group_runner.dart
index ce58eda8bd8e0f15b9d9a4348a57062d2e68e81f..e609ba22bbe6fa30d09fd1e76ddf50af27002c98 100644
--- a/pkg/barback/lib/src/graph/group_runner.dart
+++ b/pkg/barback/lib/src/graph/group_runner.dart
@@ -50,8 +50,8 @@ class GroupRunner {
   Stream<LogEntry> get onLog => _onLogPool.stream;
   final _onLogPool = new StreamPool<LogEntry>.broadcast();
 
-  GroupRunner(AssetCascade cascade, this._group, this._location) {
-    _addPhase(new Phase(cascade, _location), []);
+  GroupRunner(Phase previous, this._group, this._location) {
+    _addPhase(previous.addPhase(_location), []);
     for (var phase in _group.phases) {
       _addPhase(_phases.last.addPhase(), phase);
     }
@@ -78,11 +78,6 @@ class GroupRunner {
     }
   }
 
-  /// Adds a new asset as an input for this group.
-  void addInput(AssetNode node) {
-    _phases.first.addInput(node);
-  }
-
   /// Removes this group and all sub-phases within it.
   void remove() {
     _onLogPool.close();