src: use ZCtxt as a source for v8::Isolates

Other methods like `After` already use ZCtxt as the source for
Enviroment/ v8::Isolate objects, this commit applies the same style
to the other missing methods (`After sync` and `Write`)

PR-URL: https://github.com/nodejs/node/pull/2547


Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>