Support untyped methods

Inspiration from https://github.com/mono/mono/commit/4d4e3c3d5b273149f66abe5605e15fce11713f55

This emulates the as3 behavior with 1 exception: if you specify a
return type of '*', but not all code paths return a value, PlayScript
will consider this a compiler error instead of automatically returing
undefined in those cases. I think this behavior is favorable.