Mystery of class '[object Object]' resolved.

UndefinedObject >> subclass:... sent the symbol object
to the ClassBuilder, unlike Class which asString-ed it.

So `nil subclass: #Object` misbehaved.
Merge pull request #327 from herby/nil-subclass-symbol-fix

Mystery of class '[object Object]' resolved.
Fixes #326. Stops subtree aliasing at block boundary.
Merge pull request #328 from herby/gh-326

Fixes #326. Stops subtree aliasing at block boundary.