Sublcassing String creates non-loadable class

(see https://travis-ci.org/glassdb/glass/builds/10368234)

I subclassed String

    String subclass: #MCVersionName …

which the Browser directly turned into

    String variableSubclass: #MCVersionName …

However, the new “generated” subclass template cannot be executed anymore.
Even more, this is the format used for Monticello, hence it cannot load the class
and show-stops #2 and #3.