fix(platform-browser/testing): clean up public api for platform-browser/testing (#9519)

Mostly, removing things that were never intended to be exported publicy.

BREAKING CHANGE:

The following are no longer publicly exported APIs. They were intended as internal
utilities and you should use your own util:

```
browserDetection,
dispatchEvent,
el,
normalizeCSS,
stringifyElement,
expect (and custom matchers for Jasmine)
```

modules/@angular/compiler/test/test_component_builder_spec.ts

@@ -14,7 +14,7 @@ import {NgIf} from '@angular/common';
 import {TimerWrapper} from '../src/facade/async';
 import {IS_DART} from '../src/facade/lang';
 import {PromiseWrapper} from '../src/facade/promise';
-import {dispatchEvent} from '@angular/platform-browser/testing';
+import {dispatchEvent} from '@angular/platform-browser/testing/browser_util';
 import {withProviders} from '@angular/core/testing/test_injector';
 
 @Component(