code: function cloneCounterValues(counters) {
          const result = {};
          Object.keys(counters).forEach(name => {
              result[name] = Array.from(counters[name]);
          });
          return result;
      }
docs: Clone counter values.
  @param {!adapt.csscasc.CounterValues} counters
  @returns {!adapt.csscasc.CounterValues}