Prevent extra method.toString checks.

It appears that the intent here was to only check the
`method.toString().indexOf('_super')` once, and store the result so that
we do not need to check the same method twice, but we were never actual
storing `__hasSuper` on the method so it was just always going to check
the `toString`.
Merge pull request #5480 from rwjblue/prevent-extra-toStrings

Prevent extra method.toString checks.