Gut Chapter 4 a little bit more

Staged at:

https://ch04-dot-dart-lang.appspot.com/docs/dart-up-and-running/ch04.html

Also, https://ch04-dot-dart-lang.appspot.com/docs/dart-up-and-running/contents/ch04-tools-editor.html now redirects to /tools/editor/

R=shaza@google.com

Review URL: https://chromiumcodereview.appspot.com/690183003