Why don't we recommend installing webdev as a dev_dependency? (here).

Then we could add the constraints

dependencies:
  build_runner: ^2.4.0
  build_web_compilers: ^4.0.4

And we would not have to manually validate constraints of the current apps pubspec.lock, but rely on the pub solver to do it..

Maybe I'm missing something obvious. But it seemed to work when I tried.