save flag someplace if checksum was invalid

[catmando]

we have tests that insure that prerendering = client rendering.

Right now its a real cludge - we override console.warn, and watch to see if any checksum warnings are output.

Instead how about adding data-checksum_invalid to the outer element?

[gaearon]

We don’t have checksum warning checks now, but we still use console.error to notify about the differences.

We might add a way to hook into the warning system later. Let’s track it in #4302.