mapJSFunction function
Maps a JavaScript function to a Dart function.
jsFunctionName Name of the function.
f Dart function to map.
Implementation
void mapJSFunction(String jsFunctionName, MappedFunction f) {
  globalContext[jsFunctionName] = (JSAny a) {
    Object? r = f(a.dartify());
    return r.jsify();
  }.toJS;
}