minor copyedit of new dart2js text

Reordered things a bit (e.g. moved the Options section up to match other tools doc), converted straight quotes to curly, and tightened up the wording.

Staged at:
http://book.dart-lang.appspot.com/docs/dart-up-and-running/contents/ch04-tools-dart2js.html#ch04-tools-dart2js-generating-efficient-code

Review URL: https://chromiumcodereview.appspot.com/13835009