Better error messages when an `@Output` property is not initialized.

Related: #3754 (should be fixed together)

Listing an event in a directive without creating an associated EventEmitter instance results in cryptic error:

TypeError: Cannot read property ‘observer’ of undefined at Function.ObservableWrapper.subscribe (angular2.js:19403) at EventEmitterAccessor.subscribe (angular2.js:26062) at AppViewManagerUtils._setUpEventEmitters (angular2.js:20932) at AppViewManagerUtils._hydrateView (angular2.js:20896) at AppViewManagerUtils.hydrateRootHostView (angular2.js:20814) at AppViewManager.createRootHostView (angular2.js:23990) at angular2.js:13594 at Zone.run (zone.js:113) at Zone.run (angular2.js:12928) at zoneBoundFn (zone.js:86)

To reproduce:

@Directive({
    selector: '[foo]'
})
class Foo {
  @Output() foo; // not initialized to = new EventEmitter();
}

[luisfelix199276]

For me the error stopped when I changed the import statement from import { EventEmitter } from ‘events’; to import { EventEmitter } from ‘@angular/core’. The first option was suggested by angular as default but I think that the second one should be used. Everything works fine now after I changed it.

Example: @Output() variableToEmit: EventEmitter<string> = new EventEmitter();

showsString(text: any) { this.variableToEmit.emit(text); }

Hope that helps .

[aldo-roman]

Hi @mhevery @vicb I submitted a PR for this issue some time ago. I just fixed a conflict, lint errors and tests. See #19116

Please let me know if there is something I can do. Cheers!