Migration: fix conversion of `@required` to `required` in redirecting factory constructors.

The migration tool has logic to suppress some of its heuristics for
suggesting `required` in redirecting factory constructors, to account
for the fact that a redirecting factory constructor is allowed to have
a non-required non-nullable argument with no default.  But that logic
was overzealous and was preventing `@required` from being turned into
`required` (which is clearly still a good idea).  This pattern crops
up frequently in classes using BuiltValue.

Bug: https://b.corp.google.com/issues/234631651
Change-Id: Ic0428a878184bb837d3f5b79cb6585ec4fa30344
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/247361
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>