Bugs in JShellState#drop

[Brian Goetz]

>> We will not support forward reference of methods or classes where the
>> signature is undefined.  For a class, the signature includes the name,
>> the extends, implements, generic signature, types/names of fields,
>> name/parameter-type of methods/constructors.  What can have forward
>> references is method bodies and field initializers. Note that we will
>> attempt to corral classes whose signatures break after definition, but
>> I'm not sure the level with which that can be achieved.  We the JEP
>> for more detail on this.

What are the considerations here?  The "bright line" I was hoping for is:

  - If a declaration matches a syntactic production, but fails 
compilation, then it should be corraled.

So

   class C extends D { void foo() { moo(); } }

should be corralled even if D or moo() don't exist, but

   class C extends $(#$*# @{ void #$%()( }

would just be treated as erroneous input.