Add support for sanitizing/filtering

[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#16694

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I know that the readme talks about sanitizing being still unimplemented.

I made this task so that me and others can follow this particular work (couldn't find anything with the search).

We are using Web UI in our project and sometimes new SafeHtml.unsafe(...) even for user-supplied content which is bad, but we have to in order to support custom user-supplied HTML. Our hopes are that sooner or later we can replace those with code that sanitizes the bad input and makes sure it's clean by leveraging a whitelist approach.

[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/67

[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.

[rspilker]

We've created a separate library for white-listing tags, attributes and values that uses this library. We are willing to donate this work. Alternatively, we can work together to meet all your needs.