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/directives/ng_for_spec.ts

@@ -7,7 +7,6 @@
  */
 
 import {beforeEach, beforeEachProviders, ddescribe, describe, iit, inject, it, xit,} from '@angular/core/testing/testing_internal';
-import {expect} from '@angular/platform-browser/testing';
 import {AsyncTestCompleter} from '@angular/core/testing/testing_internal';
 import {TestComponentBuilder, ComponentFixture} from '@angular/compiler/testing';
 
@@ -16,6 +15,7 @@ import {IS_DART} from '../../src/facade/lang';
 import {Component, TemplateRef, ContentChild} from '@angular/core';
 import {NgFor} from '@angular/common';
 import {NgIf} from '@angular/common';
+import {expect} from '@angular/platform-browser/testing/matchers';
 import {By} from '@angular/platform-browser/src/dom/debug/by';
 
 export function main() {

modules/@angular/common/test/directives/ng_if_spec.ts

@@ -7,10 +7,9 @@
  */
 
 import {beforeEach, ddescribe, describe, iit, inject, it, xit,} from '@angular/core/testing/testing_internal';
-import {expect} from '@angular/platform-browser/testing';
 import {TestComponentBuilder} from '@angular/compiler/testing';
 import {AsyncTestCompleter} from '@angular/core/testing/testing_internal';
-
+import {expect} from '@angular/platform-browser/testing/matchers';
 import {getDOM} from '@angular/platform-browser/src/dom/dom_adapter';
 
 import {Component} from '@angular/core';