code: function findCoverageFile (source, filename) {
    for (var i = 0; i < source.length; i++) {
      var file = source[i]
      if (file.filename === filename) {
        return file
      }
    }
    return null
  }
docs: Find an existing coverage file
  
  @param {CoverageFile[]} source
  @param {string}         filename
  
  @returns {CoverageFile|null}