Why browsers neither Chrome nor Mozilla developer see sourcemap sass?

Question

gulp.js

Task

gulp.task('sass', function() {
  return gulp.src('app/sass/**/*.{sass, scss}')
  .pipe(sourcemaps.init())
  .pipe(sass({
    outputStyle: 'compact',
    includePaths: ['node_modules/susy/sass']		
  }).on("error", notify.onError()))
  .pipe(gcmq())
  .pipe(autoprefixer(['last 15 versions', '> 1%', 'ie 8', 'ie 7'], { cascade: true }))
  .pipe(sourcemaps.write('.'))
  .pipe(gulp.dest('app/css'))
  .pipe(browserSync.reload({stream: true}));
});