Development Mode requires the GWT Developer Plugin

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.

I had the same issue this morning.

Switching to super dev mode:

select your project in the Project Explorer -> Debug as -> Web Application(Super Dev Mode)

Console will say: The code server is ready.Next, visit: http://127.0.0.1:9876/

Do so and follow instructions there.

Quick recap: Firefox nuked APIs that DevMode needed in 27.0, more than a year ago; Chrome announced approximately at the same time that they were deprecating NPAPI, that DevMode needs. They first made it impossible to update extensions containing NPAPI plugins in the Chrome Web Store a year ago, then they removed NPAPI support on Linux, and now they've just disabled NPAPI in Chrome 42, and will remove support entirely in 45, to be released in September.

"Classic" DevMode is deprecated for nearly 1½ year now; it really is time to move to SuperDevMode!