favicon needs to get updated

[anders-sandholm]

https://github.com/dart-lang/dartdoc/blob/master/lib/resources/favicon.png still has the old favicon.
We should use the new one in https://github.com/chalin/site-shared-dartlang/blob/master/src/assets/dart/icon/64.png

@chalin @kwalrath