only check string literals in property-literals transformer

lib/6to5/transformation/transformers/_property-literals.js

@@ -1,10 +1,11 @@
 var esutils = require("esutils");
 var t       = require("../../types");
+var _       = require("lodash");
 
 exports.Property = function (node) {
   // ignore key literals that are valid identifiers
   var key = node.key;
-  if (t.isLiteral(key) && esutils.keyword.isIdentifierName(key.value)) {
+  if (t.isLiteral(key) && _.isString(key.value) && esutils.keyword.isIdentifierName(key.value)) {
     key.type = "Identifier";
     key.name = key.value;
     delete key.value;