Dartium fixes for latest Blink rolls.

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 7874d5c77c1fb86f289ac51d692051514695ea9e..a45122ab68f53c799e218fe1167cf95a05a131f3 100644
--- a/tools/dom/templates/html/impl/impl_Window.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Window.darttemplate
@@ -292,21 +292,6 @@ $if DART2JS
 $endif
 }
 
-/**
- * Event object that is fired before the window is closed.
- *
- * The standard window close behavior can be prevented by setting the
- * [returnValue]. This will display a dialog to the user confirming that they
- * want to close the page.
- */
-abstract class BeforeUnloadEvent implements Event {
-  /**
-   * If set to a non-null value, a dialog will be presented to the user
-   * confirming that they want to close the page.
-   */
-  String returnValue;
-}
-
 class _BeforeUnloadEvent extends _WrappedEvent implements BeforeUnloadEvent {
   String _returnValue;