fix bugs in docs

src/site/_includes/library-tour/html-websockets.markdown

 ### WebSockets {#html-websockets}
 
 A WebSocket allows your web app to exchange data with a server interactively—no
 polling necessary.
 A server creates the WebSocket and listens for requests on
 a URL that starts with **ws://**—for example,
 ws://127.0.0.1:1337/ws.
 The data transmitted over a WebSocket can be
 a string, a blob, or an
 [ArrayBuffer](http://api.dartlang.org/html/ArrayBuffer.html).
@@ skipping 59 matching lines @@
 Here's an example of a method that
 creates a WebSocket object
 and handles message, open, close, and error events.
 
 {% highlight dart %}
 connectToWebSocket([int retrySeconds = 2]) {
   bool encounteredError = false;
   webSocket = new WebSocket(url);
 
   webSocket.on.message.add((e) {
-    receivedMessage(e.data);
+    receivedMessage((e as MessageEvent).data);
   });
 
   webSocket.on.open.add((e) {
     print('Connected');
   });
 
   webSocket.on.close.add((e) {
     print('web socket closed, retrying in $retrySeconds seconds');
     if (!encounteredError) {
       window.setTimeout(() => connectToWebSocket(retrySeconds*2),
                                                  1000*retrySeconds);
     }
     encounteredError = true;
   });
 
   webSocket.on.error.add((e) {
     chatWindow.displayNotice('Error connecting to ws');
     if (!encounteredError) {
       window.setTimeout(() => connectToWebSocket(retrySeconds*2),
                                                  1000*retrySeconds);
     }
     encounteredError = true;
   });
 }
 {% endhighlight %}