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>