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 all the warnings, I think, 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)
* moving images into a less buried subdirectory

Please let me know (1) whether the conversion introduced errors or problems and (2) if you have ideas for ways to improve the formatting.