feat(core): add support for ShadowDOM v1 (#24718)

add a new ViewEncapsulation.ShadowDom option that uses the v1 Shadow DOM API to provide style encapsulation.

PR Close #24718

packages/examples/core/ts/metadata/encapsulation.ts

@@ -0,0 +1,35 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {Component, ViewEncapsulation} from '@angular/core';
+
+// #docregion longform
+@Component({
+  selector: 'my-app',
+  template: `
+    <h1>Hello World!</h1>
+    <span class="red">Shadow DOM Rocks!</span>
+  `,
+  styles: [`
+    :host {
+      display: block;
+      border: 1px solid black;
+    }
+    h1 {
+      color: blue;
+    }
+    .red {
+      background-color: red;
+    }
+
+  `],
+  encapsulation: ViewEncapsulation.ShadowDom
+})
+class MyApp {
+}
+// #enddocregion