Have you started the DevMode using your src/main/webapp as the “war folder”? or in other words, is there a *.nocache.js in your src/main/webapp? In that case, this file will overwrite the one produced by the GWT compiler as called by the gwt-maven-plugin.
The *.nocache.js generated by the DevMode (when no one exists, generated by a previous GWT compilation) contains only the necessary bits to launch the DevMode, and will otherwise fail with the above-mentioned error.