Added --enable-experimental-mirrors to $dart2js transformer

Author: akserg

pubspec.yaml

@@ -44,4 +44,6 @@ transformers:
       - web/tabs/tabs_demo.html
       - web/carousel/carousel_demo.html
       - web/datepicker/datepicker_demo.html
-      - web/typeahead/typeahead_demo.html
+      - web/typeahead/typeahead_demo.html
+- $dart2js:
+    commandLineOptions: [--enable-experimental-mirrors]