plaintextflow blocks generators when run from Ant build

[JoostvanPinxten]

PlainTextFlow silently blocks the generators from creating any output, even though everything seems to be set up just fine, and the build reports success for the language and the model.

Example: 2018.3.6 MPS + MPS-Extensions example of PlainTextGen + PlainTextFlow setup that generates output in a runtime MPS (i.e., the normal developer's way). It does not, however, generate the model's output for NewLanguage when run as an Ant build.

Example.zip

Preferably, this bug should not occur without any notice on the command line. Any hint would have helped me deduce the issue that I've been struggling with for a while significantly faster.