The Dictionary class comes from the old mxGraph codebase and was introduced before ES6. Its main goal is to mimic what native JavaScript Map now provides:

We should remove this class and replace its usage with Map.

The only extra feature provided by Dictionary is that remove(key) returns the removed element (if any).
In Map, this can be replaced by:

const value = map.get(key);
map.delete(key);
return value;

This usage is rare in the codebase and should be easy to migrate.