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

Reported in https://github.com/nodejs/node/pull/2411#issuecomment-152220348