Update the style guide

Staged at:
https://sg-dot-dart-lang.appspot.com/articles/style-guide/

Fixes these issues:
https://github.com/dart-lang/www.dartlang.org/issues/964
https://github.com/dart-lang/www.dartlang.org/issues/878
https://github.com/dart-lang/www.dartlang.org/issues/874
https://github.com/dart-lang/www.dartlang.org/issues/862
https://github.com/dart-lang/www.dartlang.org/issues/831
https://github.com/dart-lang/www.dartlang.org/issues/806
https://github.com/dart-lang/www.dartlang.org/issues/754
https://github.com/dart-lang/www.dartlang.org/issues/707
https://github.com/dart-lang/www.dartlang.org/issues/671
https://github.com/dart-lang/www.dartlang.org/issues/668
https://github.com/dart-lang/www.dartlang.org/issues/661
https://github.com/dart-lang/www.dartlang.org/issues/658

R=rnystrom@google.com

Review URL: https://chromiumcodereview.appspot.com/593203002