Adding API to remove send ports from Window.

tools/dom/templates/html/impl/impl_Window.darttemplate

Index: tools/dom/templates/html/impl/impl_Window.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_Window.darttemplate b/tools/dom/templates/html/impl/impl_Window.darttemplate
index 4c78373acbd2b8834a8ed34fbcdba02983461557..2e9e57b0496c6fdc82fb7df23a9da85b76d27367 100644
--- a/tools/dom/templates/html/impl/impl_Window.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Window.darttemplate
@@ -48,6 +48,15 @@ $endif
   }
 
   /**
+   * 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.
    *
@@ -283,9 +292,9 @@ $!MEMBERS
   }
 
 $if DART2JS
-  int get scrollX => JS('bool', '("scrollX" in #)', this) ? JS('int', 
+  int get scrollX => JS('bool', '("scrollX" in #)', this) ? JS('int',
       '#.scrollX', this) : document.documentElement.scrollLeft;
-  int get scrollY => JS('bool', '("scrollY" in #)', this) ? JS('int', 
+  int get scrollY => JS('bool', '("scrollY" in #)', this) ? JS('int',
       '#.scrollY', this) : document.documentElement.scrollTop;
 $endif
 }