src: fix add-on builds, partially revert 8aed9d66

Commit 8aed9d66 ("src: cleanup `Isolate::GetCurrent()`") breaks building
add-ons because of the following:

    In file included from ../node_modules/nan/nan.h:27:0,
                     from ../src/binding.cc:18:
    /home/bnoordhuis/src/v1.x/src/node_object_wrap.h: In member function
    'v8::Local<v8::Object> node::ObjectWrap::handle()':
    /home/bnoordhuis/src/v1.x/src/node_object_wrap.h:39:46: error: base
    operand of '->' has non-pointer type 'v8::Persistent<v8::Object>'
         return v8::Local<v8::Object>::New(handle_->GetIsolate(),
                                           persistent());

Mea culpa, I was one of the reviewers.

PR-URL: https://github.com/iojs/io.js/pull/868


Reviewed-By: Vladimir Kurchatkin <vladimir.kurchatkin@gmail.com>