Live HTML updating stops working after viewing CSS file

[peterflynn]

1. Open Getting Started
2. Select index.html and launch Live Preview
3. Type in the h2 tag near the top
4. Switch to main.css
5. Switch back to index.html
6. Type in the h2 tag again

Result: although the lightning bolt is still yellow, the page no longer updates. (If you continue editing, sometimes the mapping ids start to show up in the visible text of the webpage after this).

This was working in 0.42. I didn't test 0.43, but even if it was broken then too this is such a major failure in a headliner feature that I think we should treat it as a High for 0.44 -- labeling as such for now.

[redmunds]

This is also broken in 0.43. git bisect points to commit e02e71d (pull request #8605).

[peterflynn]

The console shows ConsoleAgent: data-brackets-id=74 not found in :25 after you hit this, indicating the HTML id mapping has broken down.