Allow Window.open to return null in dart2js

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 8acae64c58bef68af8114e88304584654eec0e45..883ae88ff1caaa01e4fb046abc37f08ea4f1ee66 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -31962,10 +31962,11 @@ class Window extends EventTarget implements WindowEventHandlers, WindowBase, Glo
    */
   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.