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/selector_spec.ts

@@ -10,7 +10,7 @@ import {SelectorMatcher} from '@angular/compiler/src/selector';
 import {CssSelector} from '@angular/compiler/src/selector';
 import {beforeEach, ddescribe, describe, expect, iit, it, xit} from '@angular/core/testing';
 import {getDOM} from '@angular/platform-browser/src/dom/dom_adapter';
-import {el} from '@angular/platform-browser/testing';
+import {el} from '@angular/platform-browser/testing/browser_util';
 
 export function main() {
   describe('SelectorMatcher', () => {