I've added two variations. Patch 3 renames the new interace to Callback, and fixes some javadoc.
Patch 4 parameterizes the failure type. I think this is the right thing to do, but notice that I had to fix a number of tests that had not filled in the type param on AsyncCallback to keep things compiling. (It seems odd that this was required, I wonder if it's an Open JDK bug?) What do you think of the change in and of itself? And does it count as a breaking change when what was a warning becomes a compile error? I don't think so. http://gwt-code-reviews.appspot.com/1387801/ -- http://groups.google.com/group/Google-Web-Toolkit-Contributors
