better support for Identifier in ForOfStatement

lib/6to5/templates/for-of.js

@@ -1,3 +1,3 @@
 for (var ITERATOR_KEY = OBJECT[Symbol.iterator](), STEP_KEY; !(STEP_KEY = ITERATOR_KEY.next()).done; ) {
-  var KEY = STEP_KEY.value;
+
 }

lib/6to5/transformers/for-of.js

@@ -1,31 +1,34 @@
 var util = require("../util");
+var b    = require("ast-types").builders;
 
 exports.ForOfStatement = function (node, parent, opts, generateUid) {
   var left = node.left;
-  var key;
+  var declar;
+
+  var stepKey = b.identifier(generateUid("step"));
+  var stepValueId = b.memberExpression(stepKey, b.identifier("value"), false);
 
   if (left.type === "Identifier") {
-    key = left;
+    declar = b.expressionStatement(b.assignmentExpression("=", left, stepValueId));
   } else if (left.type === "VariableDeclaration") {
-    key = left.declarations[0].id;
+    declar = b.variableDeclaration(left.kind, [
+      b.variableDeclarator(left.declarations[0].id, stepValueId)
+    ]);
   } else {
     return;
   }
 
   var node2 = util.template("for-of", {
     ITERATOR_KEY: generateUid("iterator"),
-    STEP_KEY:     generateUid("step"),
-    OBJECT:       node.right,
-    KEY:          key
+    STEP_KEY:     stepKey,
+    OBJECT:       node.right
   });
 
   util.ensureBlock(node);
 
   var block = node2.body;
+  block.body.push(declar);
   block.body = block.body.concat(node.body.body);
 
-  var declar  = block.body[0];
-  declar.kind = declar.kind;
-
   return node2;
 };