Enable the ESLint no-var rule in the external/builder/ folder

As part of testing this, I've diffed the output of `gulp mozcentral` with/without this patch and the *only* difference is the incremented `version`/`build` numbers.

external/builder/.eslintrc

@@ -0,0 +1,10 @@
+{
+  "extends": [
+    "../.eslintrc"
+  ],
+
+  "rules": {
+    // ECMAScript 6
+    "no-var": "error",
+  },
+}

external/builder/builder.js

@@ -33,6 +33,11 @@ const fs = require("fs"),
  * //#endif
  */
 function preprocess(inFilename, outFilename, defines) {
+  let lineNumber = 0;
+  function loc() {
+    return fs.realpathSync(inFilename) + ":" + lineNumber;
+  }
+
   // TODO make this really read line by line.
   const lines = fs.readFileSync(inFilename).toString().split("\n");
   const totalLines = lines.length;
@@ -118,10 +123,7 @@ function preprocess(inFilename, outFilename, defines) {
   let state = STATE_NONE;
   const stack = [];
   const control = /^(?:\/\/|<!--)\s*#(if|elif|else|endif|expand|include|error)\b(?:\s+(.*?)(?:-->)?$)?/;
-  let lineNumber = 0;
+
-  var loc = function () {
-    return fs.realpathSync(inFilename) + ":" + lineNumber;
-  };
   while ((line = readLine()) !== null) {
     ++lineNumber;
     const m = control.exec(line);
@@ -277,11 +279,11 @@ exports.preprocessCSS = preprocessCSS;
  * the first.
  */
 function merge(defaults, defines) {
-  const ret = {};
+  const ret = Object.create(null);
-  for (var key in defaults) {
+  for (const key in defaults) {
     ret[key] = defaults[key];
   }
-  for (key in defines) {
+  for (const key in defines) {
     ret[key] = defines[key];
   }
   return ret;

external/builder/preprocessor2.js

@@ -34,14 +34,14 @@ function handlePreprocessorAction(ctx, actionName, args, loc) {
         if (!arg || arg.type !== "Literal" || typeof arg.value !== "string") {
           throw new Error("No code for testing is given");
         }
-        var isTrue = !!evalWithDefines(arg.value, ctx.defines);
+        const isTrue = !!evalWithDefines(arg.value, ctx.defines);
         return { type: "Literal", value: isTrue, loc };
       case "eval":
         arg = args[0];
         if (!arg || arg.type !== "Literal" || typeof arg.value !== "string") {
           throw new Error("No code for eval is given");
         }
-        var result = evalWithDefines(arg.value, ctx.defines);
+        const result = evalWithDefines(arg.value, ctx.defines);
         if (
           typeof result === "boolean" ||
           typeof result === "string" ||
@@ -62,14 +62,14 @@ function handlePreprocessorAction(ctx, actionName, args, loc) {
         if (!arg || arg.type !== "Literal" || typeof arg.value !== "string") {
           throw new Error("Path to JSON is not provided");
         }
-        var jsonPath = arg.value;
+        let jsonPath = arg.value;
         if (jsonPath.indexOf(ROOT_PREFIX) === 0) {
           jsonPath = path.join(
             ctx.rootPath,
             jsonPath.substring(ROOT_PREFIX.length)
           );
         }
-        var jsonContent = fs.readFileSync(jsonPath).toString();
+        const jsonContent = fs.readFileSync(jsonPath).toString();
         const parsedJSON = acorn.parse("(" + jsonContent + ")", {
           ecmaVersion: ACORN_ECMA_VERSION,
         });
@@ -175,7 +175,7 @@ function postprocessNode(ctx, node) {
               case "string":
               case "boolean":
               case "number":
-                var equal = node.left.value === node.right.value;
+                const equal = node.left.value === node.right.value;
                 return {
                   type: "Literal",
                   value: (node.operator[0] === "=") === equal,
@@ -210,7 +210,7 @@ function postprocessNode(ctx, node) {
       }
       break;
     case "BlockStatement":
-      var subExpressionIndex = 0;
+      let subExpressionIndex = 0;
       while (subExpressionIndex < node.body.length) {
         switch (node.body[subExpressionIndex].type) {
           case "EmptyStatement":
@@ -219,7 +219,7 @@ function postprocessNode(ctx, node) {
             continue;
           case "BlockStatement":
             // Block statements inside a block are moved to the parent one.
-            var subChildren = node.body[subExpressionIndex].body;
+            const subChildren = node.body[subExpressionIndex].body;
             Array.prototype.splice.apply(
               node.body,
               [subExpressionIndex, 1].concat(subChildren)
@@ -240,7 +240,7 @@ function postprocessNode(ctx, node) {
       break;
     case "FunctionDeclaration":
     case "FunctionExpression":
-      var block = node.body;
+      const block = node.body;
       if (
         block.body.length > 0 &&
         block.body[block.body.length - 1].type === "ReturnStatement" &&
@@ -292,7 +292,7 @@ function fixComments(ctx, node) {
 function traverseTree(ctx, node) {
   // generic node processing
   for (const i in node) {
-    var child = node[i];
+    const child = node[i];
     if (typeof child === "object" && child !== null && child.type) {
       const result = traverseTree(ctx, child);
       if (result !== child) {