Add a dataTransfer member to MouseEvent

tools/dom/templates/html/dartium/impl_MouseEvent.darttemplate

Index: tools/dom/templates/html/dartium/impl_MouseEvent.darttemplate
diff --git a/tools/dom/templates/html/dartium/impl_MouseEvent.darttemplate b/tools/dom/templates/html/dartium/impl_MouseEvent.darttemplate
index 229afe623fc25ff829e3616e5e8af12b94720d86..cc263cf4f56c283a4557251c79a7d3612b9fee31 100644
--- a/tools/dom/templates/html/dartium/impl_MouseEvent.darttemplate
+++ b/tools/dom/templates/html/dartium/impl_MouseEvent.darttemplate
@@ -72,4 +72,8 @@ $!MEMBERS
   @DomName('MouseEvent.pageX')
   @DomName('MouseEvent.pageY')
   Point get page => new Point/*<num>*/(_pageX, _pageY);
+
+  @DomName('MouseEvent.dataTransfer')
+  DataTransfer get dataTransfer => js.JsNative.getProperty(js.context, 'dataTransfer');
+
 }