add let-scoping-return template to let scoping transformer

lib/6to5/templates/let-scoping-return.js

@@ -0,0 +1 @@
+if (typeof RETURN === "object") return RETURN.v;

lib/6to5/transformation/transformers/let-scoping.js

@@ -287,10 +287,9 @@ var run = function (forParent, block, parent, file, scope) {
 
     if (has.hasReturn) {
       // typeof ret === "object"
-      retCheck = t.ifStatement(
-        t.binaryExpression("===", t.unaryExpression("typeof", ret, true), t.literal("object")),
-        t.returnStatement(t.memberExpression(ret, t.identifier("v")))
-      );
+      retCheck = util.template("let-scoping-return", {
+        RETURN: ret,
+      });
 
       // there's no `break` or `continue` so we can just push in the `if`
       if (!has.hasBreak && !has.hasContinue) {