angular 2: when resolving promise in component via service 'this' is 'window'

Question

I'm following the quickstart tutorial for Angular 2 (https://angular.io/docs/ts/latest/tutorial/toh-pt4.html#!#review-the-app-structure) and I got stuck in the Services chapter. This is my component:

@Component({
  selector: 'main',
  templateUrl: 'main/main.template.html',
  styleUrls: ['main/main.component.css'],

  providers: [HeroService],
  directives: [HeroComponent]
})

export class MainComponent implements OnInit {
  title: String = 'Tour of heroes';
  heroes: Hero[];
  selectedHero: Hero;

  constructor(private _heroService: HeroService) {

  }

  getHeroes() {
    this._heroService.getHeroes().then(heroes =>
      this.heroes = heroes
    );
  }

  ngOnInit() {
    this.getHeroes();
  }

  onSelect(hero: Hero) { this.selectedHero = hero; }
}

As you can see it implements OnInit, which executes the component's getHeroes method, which in turns calls the injected HeroService:

import { Injectable } from 'angular2/core';
import { HEROES } from '../hero/hero.mock';

@Injectable()
export class HeroService {
  public getHeroes() {
    return Promise.resolve(HEROES);
  }

}

The promise resolves successfully and I get the array from hero.mock.ts in the response variable:

getHeroes() {
  this._heroService.getHeroes().then(heroes =>  // heroes = Array[10]
    this.heroes = heroes
  );
}

The problem I'm facing is the first this (this._heroService) is correctly set to MainComponent, but the second one (this.heroes) is referencing the Window javascript object. I've checked several other answers, including this and done as they suggest, but the problem remains. Can anyone think of a reason why this is happening?

Edit: Generated javascript for MainComponent#getHeroes

MainComponent.prototype.getHeroes = function () {
    var _this = this;
    this._heroService.getHeroes().then(function (heroes) {
        return _this.heroes = heroes;
     });
};
MainComponent.prototype.ngOnInit = function () {
    this.getHeroes();
};

Another edit: If I change the method calling the service to this (note the curly brackets wrapping everything after the =>) then this is MainComponent, but neither the changes in the title nor in the heroes array are reflected in the view:

getHeroes() {
  this._heroService.getHeroes().then(heroes => {
    console.log(this);
    this.title = 'modified string';
    this.heroes = heroes;
  });
}