The polyfill for promise is there and is working the CLI installs and sets one up, the promise-polyfill. I think the problem is with error handling in aurelia-dialog therefore instead of getting the actual error related to aurelia-dialog a generic Promise error is shown. IDK grasping for starws.
I already had the other polyfills @ghiscoding suggested, but as stated those are for IE10 and IE9. Don’t know if they make a difference with IE11.
I found a solution, but it would be nice to know why this does not work. Is this a reproducible error. Kind of sad that one can crash and burn only after 4 or 5 steps without any documentation. Even though it is not stated anywhere I wonder if Aurelia Dialog 2.0 only works with evergreen browsers. Or what is Aurelia Dialog 2.0 dependent on that a polyfill is needed to get it to work with IE11. This might not be a problem with the code but with the documentation not stating that Dialog 2.0 will only work with evergreen browsers or if you want IE11 support then you need to do XYZ. Again IDK grasping for starws.
My solution was to roll back to Aurelia Dialog 1.1.0.
For the moment that was the only major thing holding me back, had some issues with element-closest and switched from isomorphic-fetch to whatwg-fetch. Other than that everything else works.