It's astonishing that the Dart IDE is Eclipse-based - it seemed to me that Google learned its lesson after rewriting Android Studio on IntelliJ Idea, but I've been wrong.
You have to remember that Dart Editor was started before Google moved to IntelliJ for Android (at least publicly). Dart Lang is a relatively small team compared to the people working on the Android dev stuff, so my guess is that it probably was a matter of not dividing resources for an unnecessary transition. In my experience Dart Editor works quite well. Of course that's just anecdotal... Perhaps a Google engineer can shed more light on this matter.
