Have postMessage()s throw TypeError on failed arity checks.

LayoutTests/fast/workers/resources/worker-thread-multi-port.js

 onmessage = function(event) {
     if (event.data == "noport") {
         if (event.ports && !event.ports.length)
             testPassed("event.ports is non-null and zero length when no port sent");
         else
             testFailed("event.ports is null or non-zero length when no port sent");
     } else if (event.data == "zero ports") {
         if (event.ports && !event.ports.length)
             testPassed("event.ports is non-null and zero length when empty array sent");
         else
             testFailed("event.ports is null or non-zero length when empty array sent");
     } else if (event.data == "two ports") {
         if (!event.ports) {
             testFailed("event.ports should be non-null when ports sent");
             return;
         }
         if (event.ports.length == 2)
             testPassed("event.ports contains two ports when two ports sent");
         else
             testFailed("event.ports contained " + event.ports.length + " when two ports sent");
     } else if (event.data == "failed ports") {
         if (event.ports.length == 2)
             testPassed("event.ports contains two ports when two ports re-sent after error");
         else
             testFailed("event.ports contained " + event.ports.length + " when two ports re-sent after error");
     } else if (event.data == "done") {
         postMessage("done");
         event.ports[0].postMessage("done");
+    } else if (event.data == "noargs") {
+        try {
+            postMessage();
+            testFailed("postMessage() did not throw");
+        } catch (e) {
+            testPassed("postMessage() threw exception: " + e);
+        }
     } else
         testFailed("Received unexpected message: " + event.data);
 };
 
 function testPassed(msg)
 {
     postMessage("PASS"+msg);
 }
 
 function testFailed(msg)
 {
     postMessage("FAIL"+msg);
 }