| Index: packages/barback/test/transformer/mock.dart
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| diff --git a/packages/barback/test/transformer/mock.dart b/packages/barback/test/transformer/mock.dart
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| index 09c3628efaaeeb97ecf0c15a3cf643f4d15b82df..b6120e250c97fc7fb42777556a75a40a7a1d9a01 100644
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| --- a/packages/barback/test/transformer/mock.dart
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| +++ b/packages/barback/test/transformer/mock.dart
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| @@ -49,7 +49,7 @@ abstract class MockTransformer extends Transformer {
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|    ///
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|    /// Once this transformer finishes running, this is reset to a new completer,
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|    /// so it can be used multiple times.
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| -  var _started = new Completer();
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| +  var _started = new Completer<Null>();
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|  
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|    /// `true` if any transforms are currently running.
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|    ///
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| 
 |