Address comments from CL 125123002 and additional cleanup.

dart/site/try/iframe.js

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 function dartPrint(msg) {
   window.parent.postMessage(String(msg), "*");
 }
 
 window.onerror = function (message, url, lineNumber) {
   window.parent.postMessage(
       ["error", {message: message, url: url, lineNumber: lineNumber}], "*");
 };
 
 function onMessageReceived(event) {
   var data = event.data;
   if (data instanceof Array) {
     if (data.length == 2 && data[0] == 'source') {
       var script = document.createElement('script');
       script.innerHTML = data[1];
       script.type = 'application/javascript';
       document.head.appendChild(script);
       return;
     }
   }
 }
 
 window.addEventListener("message", onMessageReceived, false);
 
 (function () {
-function postScrollHeight() {
-  window.parent.postMessage(["scrollHeight", document.documentElement.scrollHeight], "*");
-}
+  function postScrollHeight() {
+    window.parent.postMessage(
+      ["scrollHeight", document.documentElement.scrollHeight], "*");
+  }
 
-var observer = new (window.MutationObserver||window.WebKitMutationObserver||window.MozMutationObserver)(function(mutations) {
-  postScrollHeight()
-  window.setTimeout(postScrollHeight, 500);
-});
+  var mutationObserverConstructor =
+      window.MutationObserver ||
+      window.WebKitMutationObserver ||
+      window.MozMutationObserver;
 
-observer.observe(
-    document.body,
-    { attributes: true,
-      childList: true,
-      characterData: true,
-      subtree: true });
+  var observer = new mutationObserverConstructor(function(mutations) {
+    postScrollHeight()
+    window.setTimeout(postScrollHeight, 500);
+  });
+
+  observer.observe(
+      document.body,
+      { attributes: true,
+        childList: true,
+        characterData: true,
+        subtree: true });
 })();