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/testing/mock_backend.ts

@@ -23,7 +23,8 @@ import {Response} from '../src/static_response';
  *
  * Mock Connection to represent a {@link Connection} for tests.
  *
- **/
+ * @experimental
+ */
 export class MockConnection implements Connection {
   // TODO Name `readyState` should change to be more generic, and states could be made to be more
   // descriptive than XHR states.
@@ -141,7 +142,9 @@ export class MockConnection implements Connection {
  * ```
  *
  * This method only exists in the mock implementation, not in real Backends.
- **/
+ *
+ * @experimental
+ */
 @Injectable()
 export class MockBackend implements ConnectionBackend {
   /**