Added some more documentation to top level Window.

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index 26dcd4a644867c601399fdad5d5bb06aa9246936..7d1228c7ba119388293bfd0114aba04307166da4 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -35,6 +35,19 @@ import 'dart:web_audio' as web_audio;
 
 Window _window;
 
+/**
+ * Top-level container for a web page, which is usually a browser tab or window.
+ *
+ * Each web page loaded in the browser has its own [Window], which is a
+ * container for the web page.
+ *
+ * This getter usually retuns the only [Window] on the page, but any `<iframe>`
+ * elements have their own [Window].
+ *
+ * See also:
+ *
+ *   * [Window](https://developer.mozilla.org/en-US/docs/Web/API/window) from MDN.
+ */
 Window get window {
   if (_window != null) {
     return _window;
@@ -45,6 +58,9 @@ Window get window {
 
 HtmlDocument _document;
 
+/**
+ * Root node for all content in a web page.
+ */
 HtmlDocument get document {
   if (_document != null) {
     return _document;