Previously, we would generate a stub for a redirecting factory, but
the default parameter values were not correctly forwarded.
This could have been fixed by copying the default parameter values into
the stub, but this conflicts with modular compilation, since the
element model does not expose default parameter values, and we should
not rely on the AST of libraries not part of the current build unit.
Apart from fixing this bug, this is also more aligned with how all the
backends actually work.
R=kmillikin@google.com
Review URL: https://chromereviews.googleplex.com/513577013 .
Peter von der Ahé