optional for-of transformer that puts arrays in fast path

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

@@ -0,0 +1,13 @@
+for (var LOOP_OBJECT = OBJECT,
+         IS_ARRAY = Array.isArray(LOOP_OBJECT),
+         INDEX = 0,
+         LOOP_OBJECT = IS_ARRAY ? LOOP_OBJECT : LOOP_OBJECT[Symbol.iterator]();;) {
+  if (IS_ARRAY) {
+    if (INDEX >= LOOP_OBJECT.length) break;
+    ID = LOOP_OBJECT[INDEX++];
+  } else {
+    INDEX = LOOP_OBJECT.next();
+    if (INDEX.done) break;
+    ID = INDEX.value;
+  }
+}

lib/6to5/transformation/transform.js

@@ -68,6 +68,7 @@ _.each({
   computedPropertyNames:     require("./transformers/es6-computed-property-names"),
   destructuring:             require("./transformers/es6-destructuring"),
   defaultParameters:         require("./transformers/es6-default-parameters"),
+  forOfFast:                 require("./transformers/optional-for-of-fast"),
   forOf:                     require("./transformers/es6-for-of"),
   unicodeRegex:              require("./transformers/es6-unicode-regex"),
   abstractReferences:        require("./transformers/es7-abstract-references"),

lib/6to5/transformation/transformers/optional-for-of-fast.js

@@ -0,0 +1,39 @@
+var util = require("../../util");
+var t    = require("../../types");
+
+exports.optional = true;
+
+exports.ForOfStatement = function (node, parent, file, scope) {
+  var left = node.left;
+  var declar, id;
+
+  if (t.isIdentifier(left)) {
+    id = left;
+  } else if (t.isVariableDeclaration(left)) {
+    id = left.declarations[0].id;
+    declar = t.variableDeclaration(left.kind, [
+      t.variableDeclarator(id)
+    ]);
+  } else  {
+    throw file.errorWithNode(left, "Unknown node type " + left.type + " in ForOfStatement");
+  }
+
+  var node2 = util.template("for-of-fast", {
+    LOOP_OBJECT:  file.generateUidIdentifier("loopObject", scope),
+    IS_ARRAY:     file.generateUidIdentifier("isArray", scope),
+    INDEX:        file.generateUidIdentifier("i", scope),
+    ID:           id,
+    OBJECT:       node.right
+  });
+
+  t.inheritsComments(node2, node);
+  t.ensureBlock(node);
+
+  var block = node2.body;
+  if (declar) {
+    block.body.unshift(declar);
+  }
+  block.body = block.body.concat(node.body.body);
+  
+  return node2;
+};