Fixed metamodel runtime bug and added assertion for JS backend to fix
Fixed metamodel runtime bug and added assertion for JS backend to fix

lucaswerkmeister/ceylon.language,unratito/ceylon.language,lucaswerkmeister/ceylon.language,unratito/ceylon.language,ceylon/ceylon.language,jvasileff/ceylon.language,jvasileff/ceylon.language,ceylon/ceylon.language