The reason is that when advancing language version, for example from 1.5
to 1.6, there is a brief period between bootstraps where the
bootstrapped compiler has latest stable language version 1.6, yet the
project is still built with 1.5. That leads to a (correct) warning that
progressive mode makes no sense unless the latest language version is
used, which fails the build because of Werror.
When testing bootstrap in progressive mode, we don't actually care about
these warnings, so disable Werror in this case.
The only case where this could matter would be if we were introducing
_warnings_ in progressive mode and checking that there are no new
warnings on Kotlin would be important for us, but so far progressive
mode has only been used to turn already existing warnings to errors.
Alexander Udalov
a383d45534