This lets us substitute different values for a type parameter depending
on the variance of its use site. For example:
(T) => T
can be substituted with [bottom <: T <: num] to get:
(bottom) => num
which is the best upper bound that doesn't use T.
This generalizes the existing substitution method, since it's
complicated enough that I don't want to maintain two versions of it.
BUG=
R=kmillikin@google.com
Review URL: https://chromereviews.googleplex.com/501577015 .
Asger Feldthaus
feded664c0