code: function summarizeCoverage(coverage) {
      var fileSummary = [];
      Object.keys(coverage).forEach(function (key) {
        fileSummary.push(summarizeFileCoverage(coverage[key]));
      });
      return mergeSummaryObjects.apply(null, fileSummary);
    }
docs: returns the coverage summary for a single coverage object. This is
  wrapper over `summarizeFileCoverage` and `mergeSummaryObjects` for
  the common case of a single coverage object
  @method summarizeCoverage
  @static
  @param {Object} coverage  the coverage object
  @return {Object} summary coverage metrics across all files in the coverage object