syscall/js: improve import functions

syscall/js: improve import functions

1. Make import functions not use the js.Value type directly,
but only the ref field. This gives more flexibility on the Go side
for the js.Value type, which is a preparation for adding
garbage collection of js.Value.

2. Turn import functions which are methods of js.Value into
package-level functions. This is necessary to make vet happy.

Change-Id: I69959bf1fbea0a0b99a552a1112ffcd0c024e9b8
Reviewed-on: https://go-review.googlesource.com/118656
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>