On a fresh install (after removing node_modules, bower_components, app/lib, dist), I still see this error:

WARN [web-server]: 404: /base/app/lib/angular-ui-router/angular-ui-router.js
PhantomJS 1.9.7 (Mac OS X) ERROR: 'There is no timestamp for /base/app/lib/angular-ui-router/angular-ui-router.js!'

PhantomJS 1.9.7 (Mac OS X) ERROR
  Error: Script error for: uiRouter
  http://requirejs.org/docs/errors.html#scripterror
  at /Users/igopinatha/workspace/git/spring-projects/spring-xd/spring-xd-ui/node_modules/requirejs/require.js:141