Adding API to remove send ports from 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 ab63b3292c9e6b59ada09b292c949344f19ba97c..091ffab0b93603015ccf59ad87d194b57091077e 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -25526,6 +25526,15 @@ class Window extends EventTarget implements WindowBase, WindowTimers {
   }
 
   /**
+   * Deregister a [port] on this window under the given [name].  This
+   * port may be retrieved by any isolate (or JavaScript script)
+   * running in this window.
+   */
+  void deregisterPort(String name) {
+    document.documentElement.attributes.remove('dart-port:$name');
+  }
+
+  /**
    * Returns a Future that completes just before the window is about to
    * repaint so the user can draw an animation frame.
    *