Things to do when we drop IE11 support someday

Writing this issue to keep track of the things we can do / drop once we no longer support IE11. This issue is mainly for record-keeping. Eventually dropping will likely be based on Microsoft ending support for Windows 7, which is [currently slated for January 2020](https://support.microsoft.com/en-us/help/13853/windows-lifecycle-fact-sheet).

- [ ] Incorporate CSS custom properties into theming
- [x] Drop extra code for sticky positioning and just use `position: sticky`
- [ ] Use `IntersectionObserver` for overlay positioning
- [ ] Use WeakSet instead of WeakMap
- [x] Second parameter to `classList.toggle`
- [x] Use `element.remove()` instead of `element.parentNode.removeChild(element)`
- [ ] Start using CSS Grid, potentially deprecating mat-grid-list
- [ ] Start using `KeyboardEvent.key` / `KeyboardEvent.code` (re-evaluate cdk/keycodes)
- [ ] Explore possibly using native Shadow DOM
- [ ] Remove re-implementations of `matches` and `closest`
- [x] Get rid of IE-specific cases in `InteractivityChecker`
- [ ] Explore `clip-path` as a means of custom shapes for components
- [x] Remove IE11-specific fallback animation in mat-spinner
- [ ] Can use `unset` CSS value

Este issue es una lista de comprobación para llevar un registro de los cambios posteriores al fin del soporte de IE11, incluidos CSS custom properties, IntersectionObserver, CSS Grid, APIs de teclado, Shadow DOM y otras funcionalidades del navegador. No se nombran archivos, pruebas ni puntos de entrada; empieza seleccionando un elemento sin marcar y localizando el código del componente relacionado. Se considera completado cuando se evalúa o completa ese elemento y se actualiza su estado en la lista de comprobación.

Escrito por el modelo de indexación a partir del texto del issue.