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/core/test/linker/dynamic_component_loader_spec.ts

@@ -18,7 +18,7 @@ import {DOCUMENT} from '@angular/platform-browser/src/dom/dom_tokens';
 import {getDOM} from '@angular/platform-browser/src/dom/dom_adapter';
 import {BaseException} from '../../src/facade/exceptions';
 import {PromiseWrapper} from '../../src/facade/promise';
-import {el} from '@angular/platform-browser/testing';
+import {el} from '@angular/platform-browser/testing/browser_util';
 
 export function main() {
   describe('DynamicComponentLoader', function() {

modules/@angular/core/test/linker/integration_spec.ts

@@ -36,7 +36,7 @@ import {TemplateRef, TemplateRef_} from '@angular/core/src/linker/template_ref';
 
 import {Renderer} from '@angular/core/src/render';
 import {IS_DART} from '../../src/facade/lang';
-import {el, dispatchEvent} from '@angular/platform-browser/testing';
+import {el, dispatchEvent} from '@angular/platform-browser/testing/browser_util';
 
 const ANCHOR_ELEMENT = /*@ts2dart_const*/ new OpaqueToken('AnchorElement');
 

modules/@angular/core/test/linker/ng_container_integration_spec.ts

@@ -13,7 +13,7 @@ import {getDOM} from '@angular/platform-browser/src/dom/dom_adapter';
 import {OpaqueToken, ViewMetadata, Component, Directive, AfterContentInit, AfterViewInit, QueryList, ContentChildren, ViewChildren, Input} from '@angular/core';
 import {NgIf} from '@angular/common';
 import {CompilerConfig} from '@angular/compiler';
-import {el} from '@angular/platform-browser/testing';
+import {el} from '@angular/platform-browser/testing/browser_util';
 
 const ANCHOR_ELEMENT = new OpaqueToken('AnchorElement');
 
@@ -216,4 +216,4 @@ class Simple {
 })
 class MyComp {
   ctxBoolProp: boolean = false;
-}
+}

modules/@angular/core/test/linker/security_integration_spec.ts

@@ -14,7 +14,7 @@ import {provide, Injectable, OpaqueToken} from '@angular/core';
 import {CompilerConfig} from '@angular/compiler';
 import {Component, ViewMetadata} from '@angular/core/src/metadata';
 import {IS_DART} from '../../src/facade/lang';
-import {el} from '@angular/platform-browser/testing';
+import {el} from '@angular/platform-browser/testing/browser_util';
 
 import {DomSanitizationService} from '@angular/platform-browser';
 
@@ -240,4 +240,4 @@ function declareTests({useJit}: {useJit: boolean}) {
           });
     });
   });
-}
+}