Fix `Directory.current` setter compatibility with `IOOverrides`

The `Directory.current` setter has a `dynamic` parameter so that it
can accept either `String` or `Directory` arguments. (This is
asymmetric with the getter, which always returns a `Directory`.)

The corresponding `IOOverrides` callback, however, assumes that the
argument is always a `String`, and `Directory.current` passed its
`dynamic` argument through unchanged.  Consequently, overriding
the `Directory.current` setter would result in a `TypeError` when
setting `Directory.current` to a `Directory` object.

Changing `IOOverrides.setCurrentDirectory` to use a `dynamic`
parameter would be a breaking change, so instead make the
`Directory.current` setter check the argument's runtime type before
passing it along.

Bug: https://github.com/dart-lang/sdk/issues/52140
Change-Id: I3c5bba6b442b314c798bd7949dfeb5eb6251dc6e
CoreLibraryReviewExempt: No API changes and VM-only.
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/336604
Reviewed-by: Brian Quinlan <bquinlan@google.com>
Reviewed-by: Lasse Nielsen <lrn@google.com>
Auto-Submit: James Lin <jamesdlin@gmail.com>
Commit-Queue: Brian Quinlan <bquinlan@google.com>