change Function export to avoid messing with istanbul

src/babel/transformation/transformers/other/flow.js

@@ -13,12 +13,12 @@ export function Class(node) {
   node.implements = null;
 }
 
-export function Function(node) {
+exports.Function = function (node) {
   for (var i = 0; i < node.params.length; i++) {
     var param = node.params[i];
     param.optional = false;
   }
-}
+};
 
 export function TypeCastExpression(node) {
   return node.expression;