in src/generate.ts [309:318]
function guessConcreteType(type: reflect.InterfaceType): reflect.Type {
const concreteClassName = type.name.substr(1); // Strip off the leading 'I'
const parts = type.fqn.split('.');
parts[parts.length - 1] = concreteClassName;
const newFqn = parts.join('.');
const newType = type.system.tryFindFqn(newFqn);
return newType && newType.extends(type) ? newType : type;
}