Adding supported checks for XsltProcessor.

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 e1340e28788fdb9e9865eb1eb867808bbfe2a2f4..ca8426c7e45906bed6a49bff8b5923315e31ebbf 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -27019,12 +27019,18 @@ class XmlSerializer native "*XMLSerializer" {
 
 @DocsEditable
 @DomName('XSLTProcessor')
+@SupportedBrowser(SupportedBrowser.CHROME)
+@SupportedBrowser(SupportedBrowser.FIREFOX)
+@SupportedBrowser(SupportedBrowser.SAFARI)
 class XsltProcessor native "*XSLTProcessor" {
 
   @DocsEditable
   factory XsltProcessor() => XsltProcessor._create();
   static XsltProcessor _create() => JS('XsltProcessor', 'new XSLTProcessor()');
 
+  /// Checks if this type is supported on the current platform.
+  static bool get supported => JS('bool', '!!(window.XSLTProcessor)');
+
   @DomName('XSLTProcessor.clearParameters')
   @DocsEditable
   void clearParameters() native;