src: wrap source before doing syntax check

This is to ensure that it is evaluated the same way it would be if it
were to be run by node or required.

Before, the following would pass if run by node, but fail if run via
the syntax check flag:

    if (true) {
      return;
    }

Now, this will pass the syntax check

PR-URL: https://github.com/nodejs/node/pull/3587


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>