Specific case for 'true' and 'false' in the mapping between Java
implementation units and Ceylon declarations source units, because each
one is in its own Java implementation file, but both are in the Boolean
Ceylon declaration file.
In order for cross-references to work well in those native cases,
corresponding java source files and ceylon source files should map
one-to-one.
At least before we find a formal and explicit way of defining this
correspondence (through annotations ?)