Assignment into a variable used as an event handler is not reactive.

[yoshikiohshima]

Describe the bug
A stackoverflow question: [(https://stackoverflow.com/questions/58226319/assignment-into-a-variable-used-as-an-event-handler-is-not-reactive)] raises an issue of a variable used as an event handler does not update.

<script>
let count = 0;
let handler = () => {};

function update() {
    if (count >= 3) {handler = () => {count = 0;}}
    count++;
}
setInterval(update, 1000);
</script>

<h1 on:click={handler}>{count}</h1>

From code, handler and count both are props and the h1 element uses, but handler does not update the on:click action.

Note that this may just be a succinct example of #3040.

To Reproduce
Visit this REPL, wait three seconds and click the number:

https://svelte.dev/repl/cb0469ba5d8241e382677ad95a659b76?version=3.12.1

[Conduitry]

Yeah I'd say this is a duplicate of #3040.