fix(aio): switch from innerText to textContent to support older browsers

`innerText` is not supported in Firefox prior to v45. In most cases (at least
the ones we are interested in), `innerText` and `textContent` work equally well,
but `textContent` is more performant (as it doesn't require a reflow).

From [MDN][1] on the differences of `innerText` vs `textContent`:

> - [...]
> - `innerText` is aware of style and will not return the text of hidden
>   elements, whereas `textContent` will.
> - As `innerText` is aware of CSS styling, it will trigger a reflow, whereas
>   `textContent` will not.
> - [...]

[1]: https://developer.mozilla.org/en-US/docs/Web/API/Node/textContent#Differences_from_innerText

Fixes #17585

aio/content/examples/setup/src/app/app.component.spec.ts

@@ -27,7 +27,7 @@ describe('AppComponent', function () {
   it('should have expected <h1> text', () => {
     fixture.detectChanges();
     const h1 = de.nativeElement;
-    expect(h1.innerText).toMatch(/angular/i,
+    expect(h1.textContent).toMatch(/angular/i,
       '<h1> should say something about "Angular"');
   });
 });

aio/content/examples/template-syntax/src/app/app.component.ts

@@ -111,7 +111,7 @@ export class AppComponent implements AfterViewInit, OnInit {
   }
 
   onSave(event: KeyboardEvent) {
-    let evtMsg = event ? ' Event target is ' + (<HTMLElement>event.target).innerText : '';
+    let evtMsg = event ? ' Event target is ' + (<HTMLElement>event.target).textContent : '';
     this.alert('Saved.' + evtMsg);
     if (event) { event.stopPropagation(); }
   }