We need observables to remove handlers safely while firing.

If theres multiple handlers that in the function that remove themselves, only half of the handlers will fire because the iteration doesn't work well with removing items from the iteration.

const observable = new Observable();
for (let i = 0; i < 100; i++) {
    const remove = observable.add(() => {
        console.log(i);
        remove();
    })
}
observable.fire();

This will fire only half of the observers because the fire iteration will remove the items as it goes (simulated run):

// This skips 1/2 of the handlers
for (const handler of this.handlers_) {
    handler();
    this.handlers_.remove(handler);
}

is the same as

// This skips 1/2 of the handlers
for (let i = 0; i < this.handlers_.length; i++) {
    handler();
    this.handlers_.remove(handler);
}

beacuse after each iteration the item that's supposed to go next is at the index i so it gets skipped.