extra nulls in error messages from barback

[DartBot]

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...