function toMinifiedCSS(files) {
    return ss.bundler.minifyCSS(files);
  }
@ngdoc method
@name bundler.default:default#toMinifiedCSS
@methodOf bundler.default:default
@function
@description
Minify JavaScript using CleanCSS

@param {Array} files Entries with file path and content to be minified
@returns {String} Minified content