Ok, now building works, but source map is not generated and source css is not visible in debugger.
In www/build/css/app.md.css last line is:
/*# sourceMappingURL=css.map */
but source map file not exist 
According to this https://www.npmjs.com/package/gulp-sass#source-maps
ionic-gulp-tasks ( https://github.com/driftyco/ionic-gulp-tasks/blob/master/sass-build/index.js ) should include extra pipe to generate sourcemap.