Refactor checks for nosideeffects extern calls to omit the Function type

The problem with 9c4a09e1b3bfecf3eb779d9cc75e1dfc3987d8f4 was that normal functions have the type `Function` which is defined in the [es3.js](https://github.com/google/closure-compiler/blob/master/externs/es3.js#L361) extern.
Re-add your earlier comment here that the name must not be defined in the source, just in the externs. I think it's a helpful comment.