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/interfaces.ts

@@ -16,11 +16,15 @@ import {URLSearchParams} from './url_search_params';
  *
  * The primary purpose of a `ConnectionBackend` is to create new connections to fulfill a given
  * {@link Request}.
+ *
+ * @experimental
  */
 export abstract class ConnectionBackend { abstract createConnection(request: any): Connection; }
 
 /**
  * Abstract class from which real connections are derived.
+ *
+ * @experimental
  */
 export abstract class Connection {
   readyState: ReadyState;
@@ -28,12 +32,18 @@ export abstract class Connection {
   response: any;  // TODO: generic of <Response>;
 }
 
-/** An XSRFStrategy configures XSRF protection (e.g. via headers) on an HTTP request. */
+/**
+ * An XSRFStrategy configures XSRF protection (e.g. via headers) on an HTTP request.
+ *
+ * @experimental
+ */
 export abstract class XSRFStrategy { abstract configureRequest(req: Request): void; }
 
 /**
  * Interface for options to construct a RequestOptions, based on
  * [RequestInit](https://fetch.spec.whatwg.org/#requestinit) from the Fetch spec.
+ *
+ * @experimental
  */
 export interface RequestOptionsArgs {
   url?: string;
@@ -52,6 +62,8 @@ export interface RequestArgs extends RequestOptionsArgs { url: string; }
 /**
  * Interface for options to construct a Response, based on
  * [ResponseInit](https://fetch.spec.whatwg.org/#responseinit) from the Fetch spec.
+ *
+ * @experimental
  */
 export type ResponseOptionsArgs = {
   // TODO: Support Blob, ArrayBuffer, JSON