make DartEditor.ini section visible on all platforms

It's a more invasive change than you'd think, due to conditional
text. (Be sure to click Windows, Linux, and then Mac at the top,
to see all 3 versions.)

Staged at:

http://editor.dart-lang.appspot.com/docs/editor/troubleshoot.html

BUG=6158

Review URL: https://chromiumcodereview.appspot.com/11744012