Strong html

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

Index: tools/dom/templates/html/impl/impl_Document.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_Document.darttemplate b/tools/dom/templates/html/impl/impl_Document.darttemplate
index 9dae6c78d4e79daaaa71f84043ba74c3b9b5a0a5..c87a9216c11269ae2455988d992688fba70e22c2 100644
--- a/tools/dom/templates/html/impl/impl_Document.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Document.darttemplate
@@ -115,4 +115,17 @@ $if DART2JS
       => JS('TreeWalker', '#.createTreeWalker(#, #, #, false)',
           this, root, whatToShow, filter);
 $endif
+
+  @DomName('Document.visibilityState')
+  @SupportedBrowser(SupportedBrowser.CHROME)
+  @SupportedBrowser(SupportedBrowser.FIREFOX)
+  @SupportedBrowser(SupportedBrowser.IE, '10')
+  @Experimental()
+$if DART2JS
+  String get visibilityState => JS('String',
+    '(#.visibilityState || #.mozVisibilityState || #.msVisibilityState ||'
+      '#.webkitVisibilityState)', this, this, this, this);
+$else
+  String get visibilityState => _visibilityState;
+$endif
 }