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-server/testing/server.ts

@@ -6,29 +6,20 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {AppModule, OpaqueToken, PLATFORM_COMMON_PROVIDERS, PLATFORM_INITIALIZER, PlatformRef, ReflectiveInjector, assertPlatform, createPlatform, getPlatform} from '@angular/core';
-import {BrowserDynamicTestModule, browserTestCompiler} from '@angular/platform-browser-dynamic/testing';
+import {AppModule, CompilerFactory, OpaqueToken, PLATFORM_COMMON_PROVIDERS, PLATFORM_INITIALIZER, PlatformRef, ReflectiveInjector, assertPlatform, createPlatform, createPlatformFactory, getPlatform} from '@angular/core';
+import {BROWSER_DYNAMIC_TEST_COMPILER_FACTORY, BrowserDynamicTestModule} from '@angular/platform-browser-dynamic/testing';
 
 import {Parse5DomAdapter} from '../src/parse5_adapter';
 
-const SERVER_TEST_PLATFORM_MARKER = new OpaqueToken('ServerTestPlatformMarker');
-
 function initServerTests() {
   Parse5DomAdapter.makeCurrent();
 }
 
-/**
- * Creates a compiler for testing
- *
- * @stable
- */
-export const serverTestCompiler = browserTestCompiler;
-
 const TEST_SERVER_PLATFORM_PROVIDERS: Array<any /*Type | Provider | any[]*/> =
     /*@ts2dart_const*/[
       PLATFORM_COMMON_PROVIDERS,
       /*@ts2dart_Provider*/ {provide: PLATFORM_INITIALIZER, useValue: initServerTests, multi: true},
-      {provide: SERVER_TEST_PLATFORM_MARKER, useValue: true}
+      {provide: CompilerFactory, useValue: BROWSER_DYNAMIC_TEST_COMPILER_FACTORY},
     ];
 
 /**
@@ -36,12 +27,8 @@ const TEST_SERVER_PLATFORM_PROVIDERS: Array<any /*Type | Provider | any[]*/> =
  *
  * @experimental API related to bootstrapping are still under review.
  */
-export function serverTestPlatform(): PlatformRef {
-  if (!getPlatform()) {
-    createPlatform(ReflectiveInjector.resolveAndCreate(TEST_SERVER_PLATFORM_PROVIDERS));
-  }
-  return assertPlatform(SERVER_TEST_PLATFORM_MARKER);
-}
+export const serverTestPlatform =
+    createPlatformFactory('serverTest', TEST_SERVER_PLATFORM_PROVIDERS);
 
 /**
  * AppModule for testing.