fix #836.

1. The case can be described as,

const listener1 = function() {
    button.removeEventListener('click', listener1);
};
const listener2 = function() {
};

button.addEventListener('click', listener1);
button.addEventListener('click', listener2);

button.dispatchEvent(clickEvent);

in listener1, the listener will remove itself. so in the current implementation, listener2 will not be invoked because tasks.length was reduced.

2. add test cases for similar cases, for example, in listener1, it will not remove itself, but remove other listener such as listener2.