| Index: pkg/barback/lib/src/phase_input.dart | 
| diff --git a/pkg/barback/lib/src/phase_input.dart b/pkg/barback/lib/src/phase_input.dart | 
| index 840b564576394416f15f321788f98b8e6bd45b98..736bd0593fde7a5f776290b3bc01895b2d013562 100644 | 
| --- a/pkg/barback/lib/src/phase_input.dart | 
| +++ b/pkg/barback/lib/src/phase_input.dart | 
| @@ -136,7 +136,7 @@ class PhaseInput { | 
| if (brandNewTransformers.isEmpty) return; | 
|  | 
| brandNewTransformers.forEach(_transformers.add); | 
| -    _adjustTransformers(); | 
| +    if (_adjustTransformersFuture == null) _adjustTransformers(); | 
| } | 
|  | 
| /// Asynchronously determines which transformers can consume [input] as a | 
|  |