Version

master for R8.

Describe the bug

Some features are not implemented in IE11, need to bring in a polyfill or use legacy way to do stuff.

• Set is not a full implementation, missing Symbol.iterator
  • Use core-js to polyfill Set
• CustomEvent.constructor is missing
  • Use document.createEvent and Event.initEvent instead

Steps to reproduce

Host Web Chat on IE11.

Expected behavior

Basic operations are working.

Additional context

[Bug]