feat(@schematics/angular): augment universal schematics to import platform-server shims

In angular/angular#40559, a new `@angular/platform-server/init`
entry-point was introduced, which can be used to initialize the server
environment by providing shims for APIs normally provided by the browser
(such as DOM globals).

If the project is using a version of Angular (and thus
`@angular/platform-server`) that is known to include this new
entry-point, the newly generated `main.server.ts` file should import it
at the top in order to ensure that the shims will be available as soon
as possible (before other direct or transitive imports that may rely on
them). See also angular/angular#40559 for more details.

NOTE:
This `universal` schematic is the base for `app-shell`,
`@nguniversal/express-engine` and `@nguniversal/hapi-engine`.

Fixes angular/angular#40559

Author: gkalpak

packages/schematics/angular/universal/files/src/__main@stripTsExtension__.ts.template

@@ -1,6 +1,15 @@
+/***************************************************************************************************
+ * Initialize the server environment - for example, adding DOM built-in types to the global scope.
+ *
+ * NOTE:
+ * This import must come before any imports (direct or transitive) that rely on DOM built-ins being
+ * available, such as `@angular/elements`.
+ */
+import '@angular/platform-server/init';
+
 <% if (hasLocalizePackage) { %>/***************************************************************************************************
  * Load `$localize` onto the global scope - used if i18n tags appear in Angular templates.
-*/
+ */
 import '@angular/localize/init';
 
 <% } %>import { enableProdMode } from '@angular/core';

packages/schematics/angular/universal/index_spec.ts

@@ -242,6 +242,14 @@ describe('Universal Schematic', () => {
     expect(tree.exists(filePath)).toEqual(true);
   });
 
+  it(`should add import to '@angular/platform-server/init' in main file`, async () => {
+    const tree = await schematicRunner.runSchematicAsync('universal', defaultOptions, appTree)
+      .toPromise();
+    const filePath = '/projects/bar/src/main.server.ts';
+    const contents = tree.readContent(filePath);
+    expect(contents).toContain('import \'@angular/platform-server/init\'');
+  });
+
   it(`should not add import to '@angular/localize' in main file when it's not a depedency`, async () => {
     const tree = await schematicRunner.runSchematicAsync('universal', defaultOptions, appTree)
       .toPromise();