This reverts commit 4aba0356, preserving
the new tests but adjusting them to assert that the old behaviour is
restored.

As expected, there were a few projects which broke because of this.
Unfortunately, in one case the breakage crosses a project boundary:
sysprof ships D-Bus introspection XML, which is consumed by mutter and
passed through gdbus-codegen.

Since sysprof cannot add this annotation without breaking its existing
users, a warning is also not appropriate.

jhbuild#41
sysprof#17
#1726