Update Dart Editor text & figures in ch01 & ch04

Staged at:

http://m3-2.dart-lang.appspot.com/docs/dart-up-and-running/contents/ch04-tools-editor.html
http://m3-2.dart-lang.appspot.com/docs/dart-up-and-running/contents/ch01.html

Review URL: https://chromiumcodereview.appspot.com/12335117