We want GopherJS 1.12-3 to have predictable behavior when building
packages regardless of the version of Go it was built with; it should
not be matching files with "// +build go1.13" constraints some times
but not other times.

Previously, ReleaseTags of the default build context were inherited
from the Go version used to build GopherJS, which may be newer than
Go 1.12.

This change makes it have ReleaseTags that go up to go1.12 always.
That corresponds to the major version of Go and its standard library
that is supported by GopherJS 1.12-3.

Fixes #969.

GitHub-Pull-Request: #970

This is the first version of GopherJS that can be built
and used with a newer major version of Go (PR #966).
Give it a new version number so it's easier to refer to it.

This change removes the build-time check that GopherJS was built
with Go version 1.12, and replaces it with a runtime check instead.

Add and document a new GopherJS-specific environment variable:

GOPHERJS_GOROOT - if set, GopherJS uses this value as the default GOROOT value,
                  instead of using the system GOROOT as the default GOROOT value

The main reason that GopherJS required Go 1.12 to be built in the
past was as a way of ensuring that the Go 1.12 standard library
code is available on disk, which GopherJS 1.12-2 needs to build
Go code. Now that the Go distribution can provided to GopherJS
with convenience via the GOPHERJS_GOROOT environment variable,
we can allow GopherJS to be built with a newer version of Go,
as long as a Go 1.12 distribution is provided via GOPHERJS_GOROOT.

Update GopherJS installation instructions to support being installed
with Go 1.12 and newer versions of Go.

The Go distribution version check is added to build.NewSession.
That check may fail, and so it was necessary to add an error return
value to the signature of build.NewSession. The build API is deemed
semi-internal, so a breaking API change is unfortunate but acceptable.

Regenerate ./compiler/natives with:

	go generate ./...

Fixes #941.

GitHub-Pull-Request: #966

This change updates the CI script to test against Go 1.12.16¹,
the latest minor release within the Go 1.12.x series.

¹ https://groups.google.com/d/msg/golang-announce/Hsw4mHYc470/WJeW5wguEgAJ

GitHub-Pull-Request: #965

Some APIs were removed in Node 12, and new "safer" APIs must be used
instead. Perhaps this implementation is not the most elegant or produces
less then ideal error messages, but at least it compiles.

While at it, added an arena type to manage and gracefully free temporary
buffers we allocate for the duration of the call.

I couldn't find any tests for the syscall module, so I wrote a small one.

Closes #935

GitHub-Pull-Request: #949

Go 1.12 has changed syscall on Darwin to start using libSystem for
forward-compatibility. That means we need to add support for more
signals to be able to run all supported tests and examples on macOS.

Many of the mappings from libc_aaa_trampoline to SYS_AAA were based on
https://github.com/golang/go/commit/a3b01440fef3d2833909f6651455924a1c86d192.

Improve error when an unsupported signal is called,
by making the signal name visible in the error message.

Regenerate natives:

	go generate github.com/gopherjs/gopherjs/compiler/natives

GitHub-Pull-Request: #940

This change updates the CI script to test against Go 1.12.9¹,
the latest minor release within the Go 1.12.x series.
It updates the fixedbugs skip table accordingly.

¹ https://groups.google.com/d/msg/golang-announce/oeMaeUnkvVE/a49yvTLqAAAJ

GitHub-Pull-Request: #939

This image is more standard and supported,
unlike¹ the previous CircleCI-specific one.

¹ https://discuss.circleci.com/t/circleci-was-unable-to-run-the-job-runner/31894/18

Fixes #937

GopherJS 1.12 is out by now, so it's safe to remove this code that
was added to deal with differences between Go 1.11.0 and Go 1.11.1.

Do so by moving the actively used code from reflect_go1111.go back
into reflect.go as it was before PR #865, and delete reflect_go111.go.

There should not be any change in behavior.

Regenerate ./compiler/natives.

Updates #862.

Introducing `syscall/js` (#899) is so significant that we update
the version.

This PR adds a new package `syscall/js`. Originally `syscall/js` is for WebAssembly, but in GopherJS
this package works as a wrapper of `github.com/gopherjs/gopherjs/js`. With this package,
developers can use `syscall/js` both for GopherJS and WebAssembly.

The implementation is basically same as `github.com/gohperjs/gopherwasm`.

Fixes #899

As time.Now() is not accurate enough, the test time_test.TestSleep is flaky.
This change disables the test.

Regenerate the virtual filesystem to include the comment addition
to /src/reflect/reflect.go from commit 84714803.

Fixes #909

Merge pull request #900 from gopherjs/go1.12

Still work in progress.

The implementation is mock.

`go tool vet` is abandoned. Use `go vet` instead.

On Darwin, syscall was instroduced at https://github.com/golang/go/commit/a3b01440fef3d2833909f6651455924a1c86d192.

This change renames syscall to avoid collision.