code: function buildCss(isRtl) {
    const mdlSrcs = gulp.src('stylesheet/mdl.scss')
        .pipe(sass.sync().on('error', sass.logError))
        .pipe(cssInlineImages({
          webRoot: 'node_modules/material-design-lite/src',
        }));
    const dialogPolyfillSrcs = gulp.src(
        'node_modules/dialog-polyfill/dialog-polyfill.css');
    let firebaseSrcs = gulp.src('stylesheet/*.css');
  
    // Flip left/right, ltr/rtl for RTL languages.
    if (isRtl) {
      firebaseSrcs = firebaseSrcs.pipe(flip.gulp());
    }
  
    const outFile = isRtl ? 'firebaseui-rtl.css' : 'firebaseui.css';
    return streamqueue({objectMode: true},
        mdlSrcs, dialogPolyfillSrcs, firebaseSrcs)
        .pipe(concatCSS(outFile))
        .pipe(cleanCSS())
        .pipe(gulp.dest(DEST_DIR));
  }
docs: Builds the CSS for FirebaseUI.
  @param {boolean} isRtl Whether to build in right-to-left mode.
  @return {*} A stream that finishes when compilation finishes.