code: function _markAsChanged(treeData, changed) {
          changed.forEach(function (filePath) {
              var objectPath = _filePathToObjectPath(treeData, filePath);
              if (objectPath) {
                  treeData = treeData.updateIn(objectPath, _addTimestamp);
              }
          });
          return treeData;
      }
docs: @private
  
  Sets/updates the timestamp on the file paths listed in the `changed` array.
  
  @param {Immutable.Map} treeData
  @param {Array.<string>} changed list of changed project-relative file paths
  @return {Immutable.Map} revised treeData