Conversion of Dart: Up and Running to Markdown

Staged at:
https://book2-dot-dart-lang.appspot.com/docs/dart-up-and-running/

Compare to:
https://www.dartlang.org/docs/dart-up-and-running/

Pay special attention to (dis)appearing text, mostly comments that are visible and shouldn't be, or text that had multiple styles (e.g. italic + code font) in the original.

I'm considering a few more changes, such as:

* improving navigation/header
* deleting the "/contents" subdirectory (and adding the necessary redirects)
* using real "Note" styles (I fixed the warnings, I hope, but didn't reformat all the other notes into `<aside>` tags
* going through all the code samples and improving their readability, somehow (many are too wide for the available space)

Please let me know (1) whether the conversion introduced errors or problems and (2) if you have ideas for ways to improve the formatting.