* When referencing the companion itself, they should use the $$INSTANCE
field, not the (null until <clinit> returns) Companion field of the
interface.
* Interface companion init blocks should be made static.
Similar to enum entry initialization, when we have a companion object
in an interface, its constructor (or clinit) initializes its state
before the instance field in corresponding interface is initialized.
So, interface companion object must be accessed via a captured object
reference (#0, or #0.this$0 for inner anonymous objects).