This question has been asked many times, but I have yet to see anyone discuss it regarding debugging GWT with Eclipse.
I started getting this error after upgrading from GWT 2.6.0 to 2.6.1 and now 2.7.0.
I can add tomcat-juhli.jar, which is now found in the bin directory, to the classpath and make the error go away, but then I may get a variety of other errors. (Jasper, Permissions, JSTL).
After many hours of moving jars from the classpath into the WEB-INF/lib folder and back again, I no longer can get the eclipse debug perspective to open.