QueryList is not a Iterable

Trying to use a `QueryList` in a `for ... of` loop in typescript yields the following compilation error:

```
Type 'QueryList<MyHero>' is not an array type or a string type.
```

Usage example:

```
@ViewChildren(MyHero) myHeroes: QueryList<MyHero>;
recallAllHeroes(){
  for (let myHero of this.myHeroes) {
    // ...
  }
}
```

Adding @vsavkin for reference.
