Extend search to also cover dartdoc contents

[mit-mit]

Likely just starting classes and members

[isoos]

How do we envision the result pages?

1. We keep the current search result pages, and display only the package as the result.

2. We extend the search result with direct link to API documentation. This would require either a more structured output from dartdoc (1667), or we need more processing.

3. Let's do (1) first, but aim for (2) in the long term?

@mit-mit, @jakobr-google ?

[mit-mit]

I think 1) will work, but probably need to try it out a bit once we have a prototype.

[isoos]

Related update: dart-lang/pana#269 (comment)