Debugging Issue in ThemedMixin

[sebilasse]

See https://github.com/dojo/widget-core/blob/master/src/mixins/Themed.ts#L165

I have no (intuitive) idea which of 80 widgets on my page is saying
"Class name "foo" not found in theme"

Could either the stack be included or the widgets name ?
E.g.

if (!themeClassName) {
  console.warn(`${this.constructor.name}: Class name "${className}" not found in theme`);
  return null;
}