draw: use type aliases for go1.8.typealias builds too

go1.8.typealias is the build tag defined by the unreleased
"Go 1.8 + type aliases" variant in the Go repo's dev.typealias branch.

Change-Id: Iff2566249ecc9a6788b78199743d2005708f1cd7
Reviewed-on: https://go-review.googlesource.com/37511
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed