Don't return outputs from a removed PhaseInput.

pkg/barback/lib/src/phase.dart

Index: pkg/barback/lib/src/phase.dart
diff --git a/pkg/barback/lib/src/phase.dart b/pkg/barback/lib/src/phase.dart
index ab39f7170ba87037bb59af38a5a20136bbf56a15..8680e6017b0e69a33c52469b1f00d0039b4e0067 100644
--- a/pkg/barback/lib/src/phase.dart
+++ b/pkg/barback/lib/src/phase.dart
@@ -281,13 +281,11 @@ class Phase {
       if (!input.isDirty) return new Future.value(new Set());
       return input.process().then(_handleOutputs);
     }));
-    outputFutures.addAll(_groups.values.map((input) {
-      if (!input.isDirty) return new Future.value(new Set());
-      return input.process().then(_handleOutputs);
+    outputFutures.addAll(_groups.values.map((group) {
+      if (!group.isDirty) return new Future.value(new Set());
+      return group.process().then(_handleOutputs);
     }));
 
-    // TODO(nweiz): handle pass-through.
-
     return Future.wait(outputFutures).then((_) {
       // Report collisions in a deterministic order.
       outputIds = outputIds.toList();