Go 1.12 / tip support

[slimsag]

This package currently is not buildable on Go tip / 1.12, due to changes in the syscall/js package:

# github.com/gopherjs/gopherwasm/js
../../../../godev/pkg/mod/github.com/gopherjs/gopherwasm@v1.0.1/js/js_wasm.go:38:17: undefined: js.Callback
../../../../godev/pkg/mod/github.com/gopherjs/gopherwasm@v1.0.1/js/js_wasm.go:40:26: undefined: js.EventCallbackFlag
../../../../godev/pkg/mod/github.com/gopherjs/gopherwasm@v1.0.1/js/js_wasm.go:43:29: undefined: js.PreventDefault
../../../../godev/pkg/mod/github.com/gopherjs/gopherwasm@v1.0.1/js/js_wasm.go:44:29: undefined: js.StopPropagation
../../../../godev/pkg/mod/github.com/gopherjs/gopherwasm@v1.0.1/js/js_wasm.go:45:29: undefined: js.StopImmediatePropagation
../../../../godev/pkg/mod/github.com/gopherjs/gopherwasm@v1.0.1/js/js_wasm.go:49:9: undefined: js.NewCallback
../../../../godev/pkg/mod/github.com/gopherjs/gopherwasm@v1.0.1/js/js_wasm.go:53:9: undefined: js.NewEventCallback

If there are no objections, I may send a PR to support this under a // +build go1.12 tag.

[slimsag]

My bad, I see that this is actually already supported it is just in the master branch and not under the v1.0.1 tag -- I'm still getting used to Go modules :)