Doubt it's "from scratch" -- a lot of projects use Eclipse as the base for their "custom editor" when in reality it's just a bunch of plugins strapped on top of the base Eclipse. (Android editor, CTools editor, etc). You probably can just install the Dart support plugins and get the same features.

Eclipse is a very robust, very flexible editor.

UPDATE - Here seems to be the plugin: https://www.dartlang.org/tools/eclipse-plugin/