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/common/test/pipes/async_pipe_spec.ts

@@ -15,7 +15,7 @@ import {WrappedValue} from '@angular/core';
 import {EventEmitter, ObservableWrapper, PromiseWrapper, TimerWrapper} from '../../src/facade/async';
 import {getDOM} from '@angular/platform-browser/src/dom/dom_adapter';
 import {PromiseCompleter} from '../../src/facade/promise';
-import {browserDetection} from '@angular/platform-browser/testing';
+import {browserDetection} from '@angular/platform-browser/testing/browser_util';
 
 export function main() {
   describe('AsyncPipe', () => {