Now printing ./build/... file when a checktransform_test fails.
Now printing ./build/... file when a checktransform_test fails.

Review URL: https://codereview.chromium.org/1075013004

google/reflectable.dart,luisvt/reflectable,google/reflectable.dart,jakemac53/reflectable,bwu-dart-contributing/reflectable,luisvt/reflectable,bwu-dart-contributing/reflectable,jakemac53/reflectable