fix(ivy): ViewRef.rootNodes not including projected nodes (#28951)

Currently if an embedded view contains projected nodes, its `rootNodes` array will include `null` instead of the root nodes inside the projection slot. This manifested itself in one of the Material unit tests where we stamp out a template and then move its `rootNodes` into the overlay container.

This PR is related to FW-1087.

PR Close #28951

packages/core/test/acceptance/template_ref_spec.ts

@@ -0,0 +1,57 @@
+/**
+ * @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, TemplateRef, ViewChild, ViewContainerRef} from '@angular/core';
+import {TestBed} from '@angular/core/testing';
+import {expect} from '@angular/platform-browser/testing/src/matchers';
+
+describe('TemplateRef', () => {
+  describe('rootNodes', () => {
+    it('should include projected nodes in rootNodes', () => {
+      @Component({
+        selector: 'menu-content',
+        template: `
+          <ng-template>
+            Header
+            <ng-content></ng-content>
+          </ng-template>
+        `,
+        exportAs: 'menuContent'
+      })
+      class MenuContent {
+        @ViewChild(TemplateRef) template !: TemplateRef<any>;
+      }
+
+      @Component({
+        template: `
+          <menu-content #menu="menuContent">
+            <button>Item one</button>
+            <button>Item two</button>
+          </menu-content>
+        `
+      })
+      class App {
+        @ViewChild(MenuContent) content !: MenuContent;
+
+        constructor(public viewContainerRef: ViewContainerRef) {}
+      }
+
+      TestBed.configureTestingModule({declarations: [MenuContent, App]});
+      const fixture = TestBed.createComponent(App);
+      fixture.detectChanges();
+
+      const instance = fixture.componentInstance;
+      const viewRef = instance.viewContainerRef.createEmbeddedView(instance.content.template);
+      const rootNodeTextContent = viewRef.rootNodes.map(node => node && node.textContent.trim())
+                                      .filter(text => text !== '');
+
+      expect(rootNodeTextContent).toEqual(['Header', 'Item one', 'Item two']);
+    });
+  });
+
+});