From GWT Website ,http://www.gwtproject.org/release-notes.html#Release_Notes_2_6_1
GWT Development Mode will no longer be available for Chrome sometime in 2014, so we improved alternate ways of debugging. There are improvements to Super Dev Mode, asserts, console logging, and error messages.
This is because of newer chrome version.Use Super Dev Mode
Other Solution is you can enable NPAPI which was disable in chrome 42 version follow the below steps
1.Open New Tab and Enter chrome://flags/#enable-npapi
2.Enable this Enable NPAPI Mac, Windows
3.Then bottom of the page click "Relunch" button.
Which is i already explained here Development Mode requires the GWT Developer Plugin
