refactor(ignoreElements): ignoreElements now returns Observable<never> (

#3410)

* refactor(ignoreElements): ignoreElements now returns Observable<never>

Per #2640 ignoreElements should resolve to Observable<never>, #3340 asks for this explicitly, there
was also another issue with tests2png that I fix here

Closes #3340, #3410

* refactor(ignoreElements): Wrong return type on type of ignoreElements

Previous change tried to add the correct return type but ended up adding
Observable<Observable<never>> which is not what was requested

#3340 and #2640

spec/support/tests2png.opts

@@ -1,4 +1,4 @@
---require spec-js/helpers/polyfills.ts
+--require spec/helpers/polyfills.ts
 --require source-map-support/register
 --require spec/helpers/tests2png/diagram-test-runner.ts
 --require spec/helpers/testScheduler-ui.ts

src/internal/operators/ignoreElements.ts

@@ -1,7 +1,7 @@
 import { Observable } from '../Observable';
 import { Operator } from '../Operator';
 import { Subscriber } from '../Subscriber';
-import { MonoTypeOperatorFunction } from '../types';
+import { OperatorFunction } from '../types';
 
 /**
  * Ignores all items emitted by the source Observable and only passes calls of `complete` or `error`.
@@ -13,8 +13,8 @@ import { MonoTypeOperatorFunction } from '../types';
  * @method ignoreElements
  * @owner Observable
  */
-export function ignoreElements<T>(): MonoTypeOperatorFunction<T> {
-  return function ignoreElementsOperatorFunction(source: Observable<T>) {
+export function ignoreElements(): OperatorFunction<any, never> {
+  return function ignoreElementsOperatorFunction(source: Observable<any>) {
     return source.lift(new IgnoreElementsOperator());
   };
 }

src/internal/patching/operator/ignoreElements.ts

@@ -11,6 +11,6 @@ import { ignoreElements as higherOrder } from '../../operators/ignoreElements';
  * @method ignoreElements
  * @owner Observable
  */
-export function ignoreElements<T>(this: Observable<T>): Observable<T> {
-  return higherOrder()(this) as Observable<T>;
+export function ignoreElements(this: Observable<any>): Observable<never> {
+  return higherOrder()(this);
 }