Start by adding it as a post-submit builder, only in the main Go repo,
and only at tip. Mostly so it's easier to see the generated code and
iterate. Once it works, it'll be turned up in the remaining places.

It turns out there's more complexity here because this is the first
case where the host OS differs from the target port, and we wish to
run the golangbuild binary on the host OS. For now, the target port
is being communicated by explicitly setting GOOS and GOARCH env vars.

For golang/go#61104.

Change-Id: I25aa946aeb16ef43b44322c25e133d19e68e6555
Reviewed-on: https://go-review.googlesource.com/c/build/+/530336
TryBot-Bypass: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>

Taking small steps so it's easier to look over the generated files.

For golang/go#61104.

Change-Id: Ib11c8c30213d1de66575089b252127903a276737
Reviewed-on: https://go-review.googlesource.com/c/build/+/533260
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Bypass: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

I thought it was Node 14 because of go.dev/cl/338730,
but it turns out the actual image still has Node 13.
To minimize new development work on release branches,
we're not going to do anything about that now.

For golang/go#61104.

Change-Id: I86d8f4ae4c1a5e0df6921326575141fa5338fc9f
Reviewed-on: https://go-review.googlesource.com/c/build/+/533264
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Bypass: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

One more gradual step. It exists in both x/arch and x/mobile now,
but only enabled in the former, as expected.

For golang/go#61104.

Change-Id: I5d2562bd7be8235797ba812f41f18f0ea4ba871a
Reviewed-on: https://go-review.googlesource.com/c/build/+/533261
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Bypass: Dmitri Shuralyov <dmitshur@golang.org>

For golang/go#61104.

Change-Id: I9f80d6b2325896e154e8775a0c2fa502eff17eac
Reviewed-on: https://go-review.googlesource.com/c/build/+/533262
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Bypass: Dmitri Shuralyov <dmitshur@golang.org>

The test can't run on platforms where exec is not implemented.
Make similar skip messages a bit more detailed and consistent.

For golang/go#61104.

Change-Id: I40cecd428cf0a7b6195aee146df6ed92deea0950
Cq-Include-Trybots: luci.golang.try:x_tools-gotip-js-wasm
Reviewed-on: https://go-review.googlesource.com/c/tools/+/533158
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>

These changes are enough to get 'GOOS=js GOARCH=wasm go test ./...'
to pass.

For golang/go#61104.

Change-Id: I78a2978a83a1026adeecc81b31bf7c2aa2db2155
Cq-Include-Trybots: luci.golang.try:x_benchmarks-gotip-js-wasm
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/533695
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Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>

It doesn't build for GOOS=js GOARCH=wasm the same reason it doesn't
build for aix and plan9: the readline dependency doesn't support it.

For golang/go#61104.

Change-Id: Ic7f45fda06c2b01555048e2e167ae0e188b2b492
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Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>

This is the last step after turning it on in postsubmit everywhere.

Inherit the previous build policy of enabling presubmit for js, wasip1
builders only in the main Go repo. Leave their postsubmit policy alone
for now. If it causes trouble, we can stop testing non-latest branches
of golang.org/x repos as configured in x/build/dashboard today.

Fixes golang/go#61104.

Change-Id: I374414fbb43d47c2d354b0d8f42d4a58baf4bc7c
Reviewed-on: https://go-review.googlesource.com/c/build/+/533263
TryBot-Bypass: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

These changes are enough to get 'GOOS=js GOARCH=wasm go test ./...'
to pass.

For golang/go#61104.

Change-Id: I78a2978a83a1026adeecc81b31bf7c2aa2db2155
Cq-Include-Trybots: luci.golang.try:x_benchmarks-gotip-js-wasm