internal/poll: confine runtime OS-checks to sendFileChunk
Ref:
https://ci.chromium.org/ui/p/golang/builders/ci/gotip-solaris-amd64/
b8732788420094473425/overview
https://build.golang.org/log/
80a05c426ceaebd0906eae80e5a3afd7e92d2f41
Change-Id: I207c0043ce729be7bbba39cf376b6d39440e7f26
Reviewed-on: https://go-review.googlesource.com/c/go/+/623055
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>