[Hydration] Detect mismatches when two `render()`d templates have similar HTML content

[nolanlawson]

Say you have two templates:

<!-- a.html -->
<template>
  <h1>a</h1>
</template>

<!-- b.html -->
<template>
  <h1>b</h1>
</template>

If you use render() to return one template on the client and another on the server, you would expect a hydration mismatch since the <h1>s have different content in this case.

However, this only works today when the static content optimization is disabled. When it's enabled (by default), we do not detect hydration mismatches and (worse) we render a even though the client wants b.

Repro: d0cd953

[nolanlawson]

This might need more investigation. It could be that this "just works" today because the validation of the <style> itself would fail. So it probably wouldn't work at all if the two templates were unstyled.