improving errors for unbalanced trees
<img src="https://avatars.githubusercontent.com/u/5479?v=3" align="left" width="96" height="96"hspace="10"> Issue by sethladd Originally opened as dart-lang/sdk#16695
(Moved from dart-lang/web-ui#217)
These are a couple minor improvements we can do to give early feedback to our users:
when there is a mismatch close tag, show the location of the open-tag that wasn't closed correctly when a component close tag is missing (either use a warning always, or at least if such component has no <content> tags in it)
<img src="https://avatars.githubusercontent.com/u/5479?v=3" align="left" width="48" height="48"hspace="10"> Comment by sethladd
Was https://github.com/dart-lang/html5lib-now-on-dart.googlecode.com/issues/49
<img src="https://avatars.githubusercontent.com/u/17034?v=3" align="left" width="48" height="48"hspace="10"> Comment by kevmoo
Added Pkg-Html5Lib label.
<img src="https://avatars.githubusercontent.com/u/17034?v=3" align="left" width="48" height="48"hspace="10"> Comment by kevmoo
Removed Library-Html5lib label.