Push substitution logic down into `_visitArguments`.

Rather than compute a preliminary `invokeType` prior to calling
`_visitArguments`, we just compute the necessary substitution to go
from `rawType` to `invokeType`, and let `_visitArguments` apply it
when needed.  This will improve efficiency when I add support for
better type inference of `fold` (see
https://github.com/dart-lang/language/issues/731), because it will
allow multiple passes of type inference without having to recompute
`invokeType` each time.  Also, it gives `_visitArguments` access to
the `rawType`, which I will take advantage of in a follow-up CL to
make constraint generation happen at a more reasonable time.

Change-Id: I2ecabac10c0ec91fc994b4cf2d8f33a5b532a99e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/238600
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>