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/platform-browser/src/browser/tools/tools.dart

@@ -15,6 +15,8 @@ import 'common_tools.dart' show AngularTools;
  * 1. Type `ng.` (usually the console will show auto-complete suggestion)
  * 1. Try the change detection profiler `ng.profiler.timeChangeDetection()`
  *    then hit Enter.
+ *
+ * @experimental All debugging apis are currently experimental.
  */
 void enableDebugTools(ComponentRef<dynamic> ref) {
   final tools = new AngularTools(ref);
@@ -29,6 +31,8 @@ void enableDebugTools(ComponentRef<dynamic> ref) {
 
 /**
  * Disables Angular 2 tools.
+ *
+ * @experimental All debugging apis are currently experimental.
  */
 void disableDebugTools() {
   context.deleteProperty('ng');

modules/@angular/platform-browser/src/browser/tools/tools.ts

@@ -23,6 +23,8 @@ var context = <any>global;
  * 1. Type `ng.` (usually the console will show auto-complete suggestion)
  * 1. Try the change detection profiler `ng.profiler.timeChangeDetection()`
  *    then hit Enter.
+ *
+ * @experimental All debugging apis are currently experimental.
  */
 export function enableDebugTools<T>(ref: ComponentRef<T>): ComponentRef<T> {
   context.ng = new AngularTools(ref);
@@ -31,6 +33,8 @@ export function enableDebugTools<T>(ref: ComponentRef<T>): ComponentRef<T> {
 
 /**
  * Disables Angular 2 tools.
+ *
+ * @experimental All debugging apis are currently experimental.
  */
 export function disableDebugTools(): void {
   delete context.ng;