Classfile artifacts to support compilation/reflection

[Brian Goetz]

>> 2.  Whether abstract classes are primitive superclass candidates.  The static compiler will check this at compilation time when it sees a superclass of a primitive class, but the JVM will want to recheck anyway.  There are two sensible ways to handle this in the classfile:
>>
>>   - An attribute that says "I am a primitive superclass candidate."  The static compiler puts it there, and the JVM         checks it.
>>   - Infer and tag.  If an abstract class is loaded that is not a primitive superclass candidate, the JVM injects IdentityObject as a superinterface of the newly loaded class; when we go to load a primitive subclass, this will fail because primitive classes cannot implement both IdentityObject and PrimitiveObject.
>>
>> Reflection probably doesn't have to reflect whether a class is primitive superclass candidate; it already reflects the things needed to make this determination.
> This one, on the other hand, conveys a core property of a JVM class.

John's notes in the SotV suggests that the JVM is comfortable just 
"figuring it out" and not requiring an attribute.  So this is the "infer 
and tag" option; the VM infers this at runtime.  Not clear if there is a 
value to having the static compiler capture something that wasn't 
explicit in the source and that has to be validated at runtime anyway.