Timeline drag-and-drop does not work in Internet Explorer

[gulench]

The drag-and-drop example, examples/timeline/other/drag&drop.html, does not work in Internet Explorer (versions ..., 10, 11).

This is the error that gets logged: SCRIPT438: Object doesn't support property or method 'includes' in lib/timeline/Core.js:

    function handleDragOver(event) {
      ...
      // make sure your target is a vis element
      if (!event.target.className.includes('vis')) return;
      ...

[yotamberk]

Works for IE11. See #2372

[mojoaxel]

@yotamberk What about IE10? Should we reopen this?

[yotamberk]

Fixes with #2426