I think I disagree:
- JavaUI.ID_CU_EDITOR is a leftover from copy-pasting code from the Java debugger into the Dart debugger. If you look at when this is returned, such a case should not/cannot happen at all, so returning there NULL as we do now, or even throwing an exception should be fine (be sure to look at getEditorInput() as well, and how getEditorInput() is called).
- But most important is that we do not have any need ATM to special case Java editors:
* For opening a Java source file (or CoffeeScript or whatever), there is a generic framework in the base Eclipse Debugging  classes; 
* For putting breakpoints in Java files - this is done "for free" by the JDI debugger, we just need to recognize the JDI breakpoints in addition to SDBGBreakpointS, but this is done in a generic way already (via IBreakpoint and friends) in BreakpotManager;