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/shadow_css_spec.ts

@@ -10,7 +10,7 @@ import {describe, beforeEach, it, expect, ddescribe, iit,} from '@angular/core/t
 import {ShadowCss, processRules, CssRule} from '@angular/compiler/src/shadow_css';
 
 import {RegExpWrapper, StringWrapper, isPresent} from '../src/facade/lang';
-import {normalizeCSS} from '@angular/platform-browser/testing';
+import {normalizeCSS} from '@angular/platform-browser/testing/browser_util';
 
 export function main() {
   describe('ShadowCss', function() {