Added top level definition to Window.

sdk/lib/html/src/CrossFrameTypes.dart

Index: sdk/lib/html/src/CrossFrameTypes.dart
diff --git a/sdk/lib/html/src/CrossFrameTypes.dart b/sdk/lib/html/src/CrossFrameTypes.dart
index 68597f09eb1a051eff88523e9f3d897fd96c225a..8f8134d62b61efc0eb31b5b32f14cad8c4a13fdb 100644
--- a/sdk/lib/html/src/CrossFrameTypes.dart
+++ b/sdk/lib/html/src/CrossFrameTypes.dart
@@ -4,6 +4,22 @@
 
 part of html;
 
+/**
+ * An object representing the top-level context object for web scripting.
+ *
+ * In a web browser, a [Window] object represents the actual browser window.
+ * In a multi-tabbed browser, each tab has its own [Window] object. A [Window]
+ * is the container that displays a [Document]'s content. All web scripting
+ * happens within the context of a [Window] object.
+ *
+ * **Note:** This class represents any window, whereas [LocalWindow] is
+ * used to access the properties and content of the current window.
+ *
+ * See also:
+ *
+ * * [DOM Window](https://developer.mozilla.org/en-US/docs/DOM/window) from MDN.
+ * * [Window](http://www.w3.org/TR/Window/) from the W3C.
+ */
 abstract class Window {
   // Fields.
   Location get location;