refactor(core): clean up platform bootstrap and initTestEnvironment

- Introduces `CompilerFactory` which can be part of a `PlatformRef`.
- Introduces `WorkerAppModule`, `WorkerUiModule`, `ServerModule`
- Introduces `serverDynamicPlatform` for applications using runtime compilation
  on the server.
- Changes browser bootstrap for runtime and offline compilation (see below for an example).
  * introduces `bootstrapModule` and `bootstrapModuleFactory` in `@angular/core`
  * introduces new `browserDynamicPlatform` in `@angular/platform-browser-dynamic
- Changes `initTestEnvironment` (which used to be `setBaseTestProviders`) to not take a compiler factory any more (see below for an example).

BREAKING CHANGE:

## Migration from `setBaseTestProviders` to `initTestEnvironment`:

- For the browser platform:
  BEFORE:
  ```
  import {setBaseTestProviders} from ‘@angular/core/testing’;
  import {TEST_BROWSER_DYNAMIC_PLATFORM_PROVIDERS,
      TEST_BROWSER_DYNAMIC_APPLICATION_PROVIDERS} from ‘@angular/platform-browser-dynamic/testing’;

  setBaseTestProviders(TEST_BROWSER_DYNAMIC_PLATFORM_PROVIDERS,
      TEST_BROWSER_DYNAMIC_APPLICATION_PROVIDERS);
  ```

  AFTER:
  ```
  import {initTestEnvironment} from ‘@angular/core/testing’;
  import {browserDynamicTestPlatform,
      BrowserDynamicTestModule} from ‘@angular/platform-browser-dynamic/testing’;

  initTestEnvironment(
      BrowserDynamicTestModule,
      browserDynamicTestPlatform());

  ```
- For the server platform:
  BEFORE:
  ```
  import {setBaseTestProviders} from ‘@angular/core/testing’;
  import {TEST_SERVER_PLATFORM_PROVIDERS,
      TEST_SERVER_APPLICATION_PROVIDERS} from ‘@angular/platform-server/testing/server’;

  setBaseTestProviders(TEST_SERVER_PLATFORM_PROVIDERS,
      TEST_SERVER_APPLICATION_PROVIDERS);
  ```

  AFTER:
  ```
  import {initTestEnvironment} from ‘@angular/core/testing’;
  import {serverTestPlatform,
      ServerTestModule} from ‘@angular/platform-browser-dynamic/testing’;

  initTestEnvironment(
      ServerTestModule,
      serverTestPlatform());

  ```

## Bootstrap changes
```
@AppModule({
  modules: [BrowserModule],
  precompile: [MainComponent],
  providers: […], // additional providers
  directives: […], // additional platform directives
  pipes: […] // additional platform pipes
})
class MyModule {
  constructor(appRef: ApplicationRef) {
    appRef.bootstrap(MainComponent);
  }
}

// offline compile
import {browserPlatform} from ‘@angular/platform-browser’;
import {bootstrapModuleFactory} from ‘@angular/core’;

bootstrapModuleFactory(MyModuleNgFactory, browserPlatform());

// runtime compile long form
import {browserDynamicPlatform} from ‘@angular/platform-browser-dynamic’;
import {bootstrapModule} from ‘@angular/core’;

bootstrapModule(MyModule, browserDynamicPlatform());
```

Closes #9922
Part of #9726

modules/@angular/platform-browser/testing/browser.ts

@@ -7,7 +7,7 @@
  */
 
 import {LocationStrategy} from '@angular/common';
-import {APP_ID, AppModule, NgZone, OpaqueToken, PLATFORM_COMMON_PROVIDERS, PLATFORM_INITIALIZER, PlatformRef, ReflectiveInjector, assertPlatform, createPlatform, getPlatform} from '@angular/core';
+import {APP_ID, AppModule, NgZone, OpaqueToken, PLATFORM_COMMON_PROVIDERS, PLATFORM_INITIALIZER, PlatformRef, ReflectiveInjector, assertPlatform, createPlatform, createPlatformFactory, getPlatform} from '@angular/core';
 
 import {BROWSER_APP_PROVIDERS, BrowserModule} from '../src/browser';
 import {BrowserDomAdapter} from '../src/browser/browser_adapter';
@@ -16,8 +16,6 @@ import {ELEMENT_PROBE_PROVIDERS} from '../src/dom/debug/ng_probe';
 
 import {BrowserDetection} from './browser_util';
 
-const BROWSER_TEST_PLATFORM_MARKER = new OpaqueToken('BrowserTestPlatformMarker');
-
 function initBrowserTests() {
   BrowserDomAdapter.makeCurrent();
   BrowserDetection.setup();
@@ -27,8 +25,13 @@ function createNgZone(): NgZone {
   return new NgZone({enableLongStackTrace: true});
 }
 
-const TEST_BROWSER_PLATFORM_PROVIDERS: Array<any /*Type | Provider | any[]*/> = [
-  PLATFORM_COMMON_PROVIDERS, {provide: BROWSER_TEST_PLATFORM_MARKER, useValue: true},
+/**
+ * Providers for the browser test platform
+ *
+ * @experimental
+ */
+export const TEST_BROWSER_PLATFORM_PROVIDERS: Array<any /*Type | Provider | any[]*/> = [
+  PLATFORM_COMMON_PROVIDERS,
   {provide: PLATFORM_INITIALIZER, useValue: initBrowserTests, multi: true}
 ];
 
@@ -37,12 +40,8 @@ const TEST_BROWSER_PLATFORM_PROVIDERS: Array<any /*Type | Provider | any[]*/> =
  *
  * @experimental API related to bootstrapping are still under review.
  */
-export function browserTestPlatform(): PlatformRef {
-  if (!getPlatform()) {
-    createPlatform(ReflectiveInjector.resolveAndCreate(TEST_BROWSER_PLATFORM_PROVIDERS));
-  }
-  return assertPlatform(BROWSER_TEST_PLATFORM_MARKER);
-}
+export const browserTestPlatform =
+    createPlatformFactory('browserTest', TEST_BROWSER_PLATFORM_PROVIDERS);
 
 /**
  * AppModule for testing.