extra nulls in error messages from barback

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="96" height="96"hspace="10"> Issue by sigmundch
Originally opened as dart-lang/sdk#20428

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Not sure where these are coming from, but I'm seeing extra nulls in the error messages from barback:

    import 'package:barback/barback.dart';
    
    class A extends Transformer {
      A.asPlugin();
      isPrimary(id) => true;
    
      apply(Transform transform) =>
        transform.getInput(new AssetId('smoke', 'lib/broken.path'));
    }

running pub-build prints these errors:

Build error:
Transform A on test|lib/transformer.dart threw error: Could not find asset smoke|lib/broken.path.
null
null
Build error:
Transform A on test|web/a.dart threw error: Could not find asset smoke|lib/broken.path.
null
null
Build failed.

I see one instead of 2 nulls if I catch the error and print the contents myself. For example:

    import 'package:barback/barback.dart';
    
    class A extends Transformer {
      A.asPlugin();
      isPrimary(id) => true;
    
      apply(Transform transform) =>
        transform.getInput(new AssetId('smoke', 'lib/broken.path'))
            .catchError((e) => print("$e"));
    }

prints:
Building test... Could not find asset smoke|lib/broken.path.
null
Could not find asset smoke|lib/broken.path.
null
Building test...