add back specNoForInOfAssignment transformer

lib/6to5/transformation/transform.js

@@ -39,6 +39,8 @@ transform.moduleFormatters = {
 };
 
 _.each({
+  specNoForInOfAssignment:   require("./transformers/spec-no-for-in-of-assignment"),
+
   // playground
   methodBinding:             require("./transformers/playground-method-binding"),
   memoizationOperator:       require("./transformers/playground-memoization-operator"),

lib/6to5/transformation/transformers/spec-no-for-in-of-assignment.js

@@ -0,0 +1,10 @@
+var t = require("../../types");
+
+exports.ForInStatement =
+exports.ForOfStatement = function (node, parent, file) {
+  var left = node.left;
+  if (t.isVariableDeclaration(left)) {
+    var declar = left.declarations[0];
+    if (declar.init) throw file.errorWithNode(declar, "No assignments allowed in for-in/of head");
+  }
+};

lib/6to5/util.js

@@ -146,10 +146,10 @@ exports.template = function (name, nodes, keepExpression) {
   var node = template.body[0];
 
   if (!keepExpression && t.isExpressionStatement(node)) {
-    node = node.expression;
+    return node.expression;
+  } else {
+    return node;
   }
-
-  return node;
 };
 
 exports.codeFrame = function (lines, lineNumber, colNumber) {

test/fixtures/transformation/spec/for-in-assignment/actual.js

@@ -0,0 +1,3 @@
+for (var i = 0 in obj) {
+
+}

test/fixtures/transformation/spec/for-in-assignment/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "No assignments allowed in for-in/of head"
+}

test/fixtures/transformation/spec/for-of-assignment/actual.js

@@ -0,0 +1,3 @@
+for (var i = 0 of obj) {
+
+}

test/fixtures/transformation/spec/for-of-assignment/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "No assignments allowed in for-in/of head"
+}