Allow Window.open to return null in dart2js

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 896e2040bcd084c7d7adfbb7e1333d10a925cf75..d891762e968cf8a5c55c2ad3f3a1b4ae2cef12aa 100644
--- a/tools/dom/templates/html/impl/impl_Window.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Window.darttemplate
@@ -47,10 +47,11 @@ $if DART2JS
    */
   Document get document => JS('Document', '#.document', this);
 
-  WindowBase _open2(url, name) => JS('Window', '#.open(#,#)', this, url, name);
+  WindowBase _open2(url, name) =>
+      JS('Window|Null', '#.open(#,#)', this, url, name);
 
   WindowBase _open3(url, name, options) =>
-      JS('Window', '#.open(#,#,#)', this, url, name, options);
+      JS('Window|Null', '#.open(#,#,#)', this, url, name, options);
 
   /**
    * Opens a new window.