Wrap Window in case it's a window from a different page / frame.

lib/html/templates/html/dart2js/impl_LocalWindow.darttemplate

Index: lib/html/templates/html/dart2js/impl_LocalWindow.darttemplate
diff --git a/lib/html/templates/html/dart2js/impl_LocalWindow.darttemplate b/lib/html/templates/html/dart2js/impl_LocalWindow.darttemplate
index 7b6d86e2ffb875784838e6e55d4d267c7cdfba16..e4cc28280399108ca655634a046cd146fe8cb177 100644
--- a/lib/html/templates/html/dart2js/impl_LocalWindow.darttemplate
+++ b/lib/html/templates/html/dart2js/impl_LocalWindow.darttemplate
@@ -6,11 +6,6 @@ class $CLASSNAME$EXTENDS$IMPLEMENTS native "@*DOMWindow" {
 
   _DocumentImpl get document() native "return this.document;";
 
-  Window get _top() native "return this.top;";
-
-  // Override top to return secure wrapper.
-  Window get top => _DOMWindowCrossFrameImpl._createSafe(_top);
-
   Window _open2(url, name) native "return this.open(url, name);";
 
   Window _open3(url, name, options) native "return this.open(url, name, options);";