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/base_request_options.ts

@@ -40,6 +40,8 @@ import {URLSearchParams} from './url_search_params';
  * console.log('req.method:', RequestMethod[req.method]); // Post
  * console.log('options.url:', options.url); // https://google.com
  * ```
+ *
+ * @experimental
  */
 export class RequestOptions {
   /**
@@ -164,6 +166,8 @@ export class RequestOptions {
  * console.log('options.url:', options.url); // null
  * console.log('req.url:', req.url); // https://google.com
  * ```
+ *
+ * @experimental
  */
 @Injectable()
 export class BaseRequestOptions extends RequestOptions {