Dartdoc customization

[isoos]

The current customization focuses on the page header:

• The icon goes to pub.dartlang.org
• The "<pkg_name> pacakge" goes to the pub.dartlang.org/packages//versions/ page
• The documentation" goes to the generated output's root (index.html)

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Submitted in separate commits for easier review.

[jakobr-google]

LGTM

[kevmoo]

FYI @jcollins-g – it might be good for you and @isoos and @jakobr-google to chat about possible tweaks to the dartdoc package. My guess is the package site will be running dartdoc more than anyone else – even the SDK 😄

[jcollins-g]

This seems suboptimal to do outside of dartdoc itself. If you have particular changes to the header generation you'd like, we should just do it there -- that way all customers of the code get the same thing. Dartdoc has template flags for headers that can be used to do some of this, at least.

[isoos]

Let's collect the requirements in an issue: #1019