don't warn about missing html, doctype, and body tags in component HTML files

[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#16693

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I don't see these warnings in the current WhatWG spec, for example: http://www.whatwg.org/specs/web-apps/current-work/multipage/tree-construction.html#the-before-head-insertion-mode

[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/69

[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.