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

此 issue 是一份用于记录 IE11 支持结束后变更的检查清单，其中包括 CSS custom properties、IntersectionObserver、CSS Grid、键盘 API、Shadow DOM 以及其他浏览器功能。未指定任何文件、测试或入口点；请先选择一个未勾选的项目，并找到其相关的组件代码。完成的标准是评估或完成该项目，并更新其检查清单状态。

由索引模型根据 Issue 内容生成。