fix(cdk/testing): avoid using dotted property access for dispatchEven… (#20754)

* fix(cdk/testing): avoid using dotted property access for dispatchEvent on ElementFinder

 * Internally this was causing an error saying that the type has a
  string index signature, but it is being accessed using a dotted property access.

* fixup! fix(cdk/testing): avoid using dotted property access for dispatchEvent on ElementFinder

Author: wagnermaciel

src/cdk/testing/protractor/protractor-element.ts

@@ -212,5 +212,6 @@ export class ProtractorElement implements TestElement {
 function _dispatchEvent(name: string, element: ElementFinder) {
   const event = document.createEvent('Event');
   event.initEvent(name);
-  element.dispatchEvent(event);
+  // This type has a string index signature, so we cannot access it using a dotted property access.
+  element['dispatchEvent'](event);
 }