(Forked off Yury Smolsky's CL 176618)
With this change, the playground.js client now asks the server to do a
vet check in the same HTTP request as the /compile (and run) step. If
the server replies that it understands the request (VetErrors or VetOK
in the repsonse), then the client can avoid the latency of a second
HTTP roundtrip. We'll remove the /vet handler after we see it fall out
of use from older clients.
Updates golang/go#31970
Change-Id: I5b123883e19cbc6a8ec30c50705e6b945a4d322d
Reviewed-on: https://go-review.googlesource.com/c/tools/+/176939
Reviewed-by: Andrew Bonventre <andybons@golang.org>