fix fast forOf and add tests

lib/6to5/transformation/transformers/es6-for-of.js

@@ -5,9 +5,10 @@ exports.ForOfStatement = function (node, parent, file, scope) {
   var callback = spec;
   if (file.isFast("forOf")) callback = fast;
 
-  var build = callback(node, parent, file, scope);
-  var loop  = build.loop;
-  var block = loop.body;
+  var build  = callback(node, parent, file, scope);
+  var declar = build.declar;
+  var loop   = build.loop;
+  var block  = loop.body;
 
   // inherit comments from the original loop
   t.inheritsComments(loop, node);
@@ -15,8 +16,14 @@ exports.ForOfStatement = function (node, parent, file, scope) {
   // ensure that it's a block so we can take all it's statemetns
   t.ensureBlock(node);
 
-  // push the value declaration to the new loop body
-  if (build.declar) block.body.push(build.declar);
+  // add the value declaration to the new loop body
+  if (declar){
+    if (build.shouldUnshift) {
+      block.body.unshift(declar);
+    } else {
+      block.body.push(declar);
+    }
+  }
 
   // push the rest of the original loop body onto our new body
   block.body = block.body.concat(node.body.body);
@@ -42,7 +49,7 @@ var fast = function (node, parent, file, scope) {
   }
 
   var loop = util.template("for-of-fast", {
-    LOOP_OBJECT:  file.generateUidIdentifier("loopObject", scope),
+    LOOP_OBJECT:  file.generateUidIdentifier("iterator", scope),
     IS_ARRAY:     file.generateUidIdentifier("isArray", scope),
     OBJECT:       node.right,
     INDEX:        file.generateUidIdentifier("i", scope),
@@ -50,8 +57,9 @@ var fast = function (node, parent, file, scope) {
   });
 
   return {
-    declar: declar,
-    loop:   loop
+    shouldUnshift: true,
+    declar:        declar,
+    loop:          loop
   };
 };