code: function _styleSheetAdded(event, url, roots) {
          var path = _server && _server.urlToPath(url),
              alreadyAdded = !!_relatedDocuments[url];
  
          // path may be null if loading an external stylesheet.
          // Also, the stylesheet may already exist and be reported as added twice
          // due to Chrome reporting added/removed events after incremental changes
          // are pushed to the browser
          if (!path || alreadyAdded) {
              return;
          }
  
          var docPromise = DocumentManager.getDocumentForPath(path);
  
          docPromise.done(function (doc) {
              if ((_classForDocument(doc) === LiveCSSDocument) &&
                      (!_liveDocument || (doc !== _liveDocument.doc))) {
                  var liveDoc = _createLiveDocument(doc, doc._masterEditor, roots);
                  if (liveDoc) {
                      _server.add(liveDoc);
                      _relatedDocuments[doc.url] = liveDoc;
                      liveDoc.on("updateDoc", function (event, url) {
                          var path = _server.urlToPath(url),
                              doc = getLiveDocForPath(path);
                          doc._updateBrowser();
                      });
                  }
              }
          });
      }
docs: Handles a notification from the browser that a stylesheet was loaded into
  the live HTML document. If the stylesheet maps to a file in the project, then
  creates a live document for the stylesheet and adds it to _relatedDocuments.
  @param {$.Event} event
  @param {string} url The URL of the stylesheet that was added.
  @param {array} roots The URLs of the roots of the stylesheet (the css files loaded through <link>)