fix: public api surface fixes + stability markers

- ts-api-guardian will now error if a new public symbol is added with a stability marker (`@stable`, `@experimental`, `@deprecated`)
- DomEventsPlugin and KeyEventsPlugin were removed from public api surface - these classes is an implementation detail
- deprecated BROWSER_PROVIDERS was removed completely
- `@angular/compiler` was removed from the ts-api-guardian check since this package shouldn't contain anything that users need to directly import
- the rest of the api surface was conservatively marked as stable or experimental

BREAKING CHANGES: DomEventsPlugin and KeyEventsPlugin previously exported from core are no longer public - these classes are implementation detail.

Previously deprecated BROWSER_PROVIDERS was completely removed from platform-browser.

Closes #9236
Closes #9235
Ref #9234

modules/@angular/http/src/headers.ts

@@ -36,6 +36,8 @@ import {isListLikeIterable, iterateListLike, Map, MapWrapper, StringMapWrapper,
  * var thirdHeaders = new Headers(secondHeaders);
  * console.log(thirdHeaders.get('X-My-Custom-Header')); //'Angular'
  * ```
+ *
+ * @experimental
  */
 export class Headers {
   /** @internal */