AFAICT it looks for the presence of a "war" folder to determine if it
should use GWTShell (tomcat, uses <servlet/> in gwt.xml) or HostedMode
(jetty, uses war/ with servlets declared in war/WEB-INF/web.xml)
Note that this is determined at the time you add the GWT nature to
your project. If you later add or remove the "war" folder, you have to
uncheck GWT in your project's option, save the project, then enable
GWT again for the change to be taken into account.