refactor(upgrade): use shared code in downgradeNg2Component() (#14037)

This unified the implementations of dynamic's `downgradeNg2Component()` and
static's `downgradeComponent()`.

modules/@angular/upgrade/test/dynamic/test_helpers.ts

@@ -7,3 +7,9 @@
  */
 
 export * from '../common/test_helpers';
+
+export function nodes(html: string) {
+  const div = document.createElement('div');
+  div.innerHTML = html.trim();
+  return Array.prototype.slice.call(div.childNodes);
+}