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/static_response.ts

@@ -32,6 +32,8 @@ import {isJsObject} from './http_utils';
  * Spec](https://fetch.spec.whatwg.org/#response-class), but is considered a static value whose body
  * can be accessed many times. There are other differences in the implementation, but this is the
  * most significant.
+ *
+ * @experimental
  */
 export class Response {
   /**