Reduce dependencies

Author: manolo

lib/com/vaadin/polymer/Elements.gwt.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <module>
     <source path="" />
-    <inherits name='com.google.gwt.user.User'/>
+    <inherits name='com.google.gwt.core.Core'/>
     <inherits name='elemental2.core.Core'/>
     <inherits name='elemental2.dom.Dom'/>
 </module>