The commit 0e4c013b changed the syscall package so it uses the
asynchronous functions of Node.js's fs module.

This commit adapts the stubs of the fs module which are used when using
a browser instead of Node.js.

Fixes #28068.

Change-Id: Ic3a6a8aebb0db06402383bc2fea7642a4501e02c
Reviewed-on: https://go-review.googlesource.com/c/140537Reviewed-by: Agniva De Sarker <agniva.quicksilver@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>