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

gulpfile.js

@@ -32,8 +32,10 @@ const entrypoints = [
   'dist/packages-dist/core/testing.d.ts',
   'dist/packages-dist/common/index.d.ts',
   'dist/packages-dist/common/testing.d.ts',
-  'dist/packages-dist/compiler/index.d.ts',
-  'dist/packages-dist/compiler/testing.d.ts',
+  // The API surface of the compiler is currently unstable - all of the important APIs are exposed
+  // via @angular/core, @angular/platform-browser or @angular/platform-browser-dynamic instead.
+  //'dist/packages-dist/compiler/index.d.ts',
+  //'dist/packages-dist/compiler/testing.d.ts',
   'dist/packages-dist/upgrade/index.d.ts',
   'dist/packages-dist/platform-browser/index.d.ts',
   'dist/packages-dist/platform-browser/testing.d.ts',
@@ -54,7 +56,7 @@ const publicApiArgs = [
   '--allowModuleIdentifiers', 'jasmine',
   '--allowModuleIdentifiers', 'protractor',
   '--allowModuleIdentifiers', 'angular',
-  '--onStabilityMissing', 'warn'
+  '--onStabilityMissing', 'error'
 ].concat(entrypoints);
 
 // Note that these two commands work on built d.ts files instead of the source