improving errors for unbalanced trees

[DartBot]

<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

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(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)

[DartBot]

<img src="https://avatars.githubusercontent.com/u/5479?v=3" align="left" width="48" height="48"hspace="10"> Comment by sethladd

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Was https://github.com/dart-lang/html5lib-now-on-dart.googlecode.com/issues/49

[DartBot]

<img src="https://avatars.githubusercontent.com/u/17034?v=3" align="left" width="48" height="48"hspace="10"> Comment by kevmoo

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Added Pkg-Html5Lib label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/17034?v=3" align="left" width="48" height="48"hspace="10"> Comment by kevmoo

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Removed Library-Html5lib label.