Allow defineClass to be called more than once with mapInheritance true

[tschaub]

If defineClass is called twice with two classes that share a common superclass, the prototype for the superclass will be created twice. This means that instanceof checks don't work (for the first class defined).

I'm not convinced that the fix in ScriptableObject is in the right pace (there's a whole lot more work going on than necessary in the case of multiple defineClass calls), but it works.

[anba]

Maybe buildClassCtor() is a better place to get this working -> anba/rhino@76adbd7 and anba/rhino@fa7e69f

[tschaub]

Thanks @anba. I agree the changes in your DefineClassMapInheritanceBug branch are a better fix. I've attached them to this pull request.

[hns]

Merged. Thanks!