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The WindowBuilder code is accessible if you know where to go... http://dev.eclipse.org/svnroot/tools/org.eclipse.windowbuilder/ If you look at the list of repositories it cannot be found though (unless I'm blind). http://dev.eclipse.org/viewcvs/viewvc.cgi?view=roots
The SVN location is listed on the project summary page... http://eclipse.org/projects/project_summary.php?projectid=tools.windowbuilder ...and now on the WindowBuilder project page... http://eclipse.org/windowbuilder/involved.php I don't know why it isn't listed in the web ui (maybe that is an incubator thing). In fact, there are lots of projects that aren't listed via that interface.
This doesn't appear to be a WindowBuilder issue. In any case, the source is linked to from the project page.