fix: better error when DARTIUM_BIN not set

Now Karma will throw:
  No binary for Dartium browser on your platform.
  Please, set "DARTIUM_BIN" env variable.

Instead of [TypeError: Cannot read property 'darwin' of undefined]

index.js

@@ -90,7 +90,7 @@ var DartiumBrowser = function(baseBrowserDecorator, args) {
 
 DartiumBrowser.prototype = {
     name: 'Dartium',
-
+    DEFAULT_CMD: {},
     ENV_CMD: 'DARTIUM_BIN'
 };