fix(fromEvent): throw if passed invalid target

Closes ReactiveX#5823

spec/observables/fromEvent-spec.ts

@@ -181,20 +181,15 @@ describe('fromEvent', () => {
     expect(offHandler).to.equal(onHandler);
   });
 
-  it('should error on invalid event targets', () => {
+  it('should throw if passed an invalid event target', () => {
     const obj = {
       addListener: () => {
         //noop
       }
     };
-
-    fromEvent(obj as any, 'click').subscribe({
-      error(err: any) {
-        expect(err).to.exist
-          .and.be.instanceof(Error)
-          .and.have.property('message', 'Invalid event target');
-      }
-    });
+    expect(() => {
+      fromEvent(obj as any, 'click');
+    }).to.throw(/Invalid event target/)
   });
 
   it('should pass through options to addEventListener and removeEventListener', () => {

src/internal/observable/fromEvent.ts

@@ -245,15 +245,13 @@ export function fromEvent<T>(
     }
   }
 
+  // If add is falsy and we made it here, it's because we didn't
+  // match any valid target objects above.
+  if (!add) {
+    throw new TypeError('Invalid event target');
+  }
+
   return new Observable<T>((subscriber) => {
-    // If add is falsy and we made it here, it's because we didn't
-    // match any valid target objects above.
-    if (!add) {
-      // TODO: We should probably discuss if throwing this at subscription-time
-      // is appropriate. It seems like it would be better (and easier to debug)
-      // to throw this when `fromEvent()` is called.
-      throw new TypeError('Invalid event target');
-    }
     // The handler we are going to register. Forwards the event object, by itself, or
     // an array of arguments to the event handler, if there is more than one argument,
     // to the consumer.