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

This issue is a record-keeping checklist for changes after IE11 support ends, including CSS custom properties, IntersectionObserver, CSS Grid, keyboard APIs, Shadow DOM, and other browser features. No files, tests, or entry points are named; start by selecting one unchecked item and locating its related component code. Done means evaluating or completing that item and updating its checklist status.

Written by the indexing model from the issue text.