Custom elements always get missing data warnings in dev mode

[Rich-Harris]

Because of the way custom elements work, if you have the traditional hello world component...

<h1>Hello {{name}}!</h1>

<script>
  export default {
    tag: 'my-app'
  };
</script>

...and create it like so...

document.body.innerHTML = `<my-app name='world'/>`;

...then at the time of creation, the name property is unpopulated (the callback fires immediately after construction).

In dev mode, this means you get incorrect warnings about missing properties.