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/upgrade/src/upgrade_adapter.ts

@@ -87,6 +87,8 @@ var upgradeCount: number = 0;
  *       "ng2[ng1[Hello World!](transclude)](project)");
  * });
  * ```
+ *
+ * @experimental
  */
 export class UpgradeAdapter {
   /* @internal */
@@ -567,6 +569,8 @@ function ng1ComponentDirective(info: ComponentInfo, idPrefix: string): Function
 
 /**
  * Use `UgradeAdapterRef` to control a hybrid AngularJS v1 / Angular v2 application.
+ *
+ * @experimental
  */
 export class UpgradeAdapterRef {
   /* @internal */