fix(keys): do not declare functions inside blocks

fixes #194

Author: vicb

lib/keys.js

@@ -2,12 +2,14 @@
  * Creates keys for `private` properties on exposed objects to minimize interactions with other codebases.
  * The key will be a Symbol if the host supports it; otherwise a prefixed string.
  */
+var create;
+
 if (typeof Symbol !== 'undefined') {
-  function create(name) {
+  create = function (name) {
     return Symbol(name);
-  } 
+  }
 } else {
-  function create(name) {
+  create = function (name) {
     return '_zone$' + name;
   }
 }
@@ -20,4 +22,4 @@ var commonKeys = {
 module.exports = {
   create: create,
   common: commonKeys
-};
+};