tools: avoid let in for loops

This adds a new ESLint tool to check for let
declarations within the for, forIn, forOf expressions.

Fixes: nodejs#9045
Ref: nodejs#8873

lib/.eslintrc

@@ -2,3 +2,4 @@ rules:
   # Custom rules in tools/eslint-rules
   require-buffer: 2
   buffer-constructor: 2
+  no-let-in-for-declaration: 2

tools/eslint-rules/no-let-in-for-declaration.js

@@ -0,0 +1,46 @@
+/**
+ * @fileoverview Prohibit the use of `let` as the loop variable
+ *               in the initialization of for, and the left-hand
+ *               iterator in forIn and forOf loops.
+ *
+ * @author Jessica Quynh Tran
+ */
+
+'use strict';
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+module.exports = {
+  create(context) {
+
+    const msg = 'Use of `let` as the loop variable in a for-loop is ' +
+                'not recommended. Please use `var` instead.';
+
+    /**
+     * Report function to test if the for-loop is declared using `let`.
+     */
+    function testForLoop(node) {
+      if (node.init && node.init.kind === 'let') {
+        context.report(node.init, msg);
+      }
+    }
+
+    /**
+     * Report function to test if the for-in or for-of loop
+     * is declared using `let`.
+     */
+    function testForInOfLoop(node) {
+      if (node.left && node.left.kind === 'let') {
+        context.report(node.left, msg);
+      }
+    }
+
+    return {
+      'ForStatement': testForLoop,
+      'ForInStatement': testForInOfLoop,
+      'ForOfStatement': testForInOfLoop
+    };
+  }
+};