Replace *most* cases of var with let/const in the external/builder/ folder

These changes were done automatically, by using the `gulp lint --fix` command, in preparation for the next patch.

external/builder/builder.js

@@ -1,6 +1,6 @@
 "use strict";
 
-var fs = require("fs"),
+const fs = require("fs"),
   path = require("path"),
   vm = require("vm");
 
@@ -34,17 +34,17 @@ var fs = require("fs"),
  */
 function preprocess(inFilename, outFilename, defines) {
   // TODO make this really read line by line.
-  var lines = fs.readFileSync(inFilename).toString().split("\n");
-  var totalLines = lines.length;
-  var out = "";
-  var i = 0;
+  const lines = fs.readFileSync(inFilename).toString().split("\n");
+  const totalLines = lines.length;
+  let out = "";
+  let i = 0;
   function readLine() {
     if (i < totalLines) {
       return lines[i++];
     }
     return null;
   }
-  var writeLine =
+  const writeLine =
     typeof outFilename === "function"
       ? outFilename
       : function (line) {
@@ -70,10 +70,10 @@ function preprocess(inFilename, outFilename, defines) {
     }
   }
   function include(file) {
-    var realPath = fs.realpathSync(inFilename);
-    var dir = path.dirname(realPath);
+    const realPath = fs.realpathSync(inFilename);
+    const dir = path.dirname(realPath);
     try {
-      var fullpath;
+      let fullpath;
       if (file.indexOf("$ROOT/") === 0) {
         fullpath = path.join(
           __dirname,
@@ -103,28 +103,28 @@ function preprocess(inFilename, outFilename, defines) {
   }
 
   // not inside if or else (process lines)
-  var STATE_NONE = 0;
+  const STATE_NONE = 0;
   // inside if, condition false (ignore until #else or #endif)
-  var STATE_IF_FALSE = 1;
+  const STATE_IF_FALSE = 1;
   // inside else, #if was false, so #else is true (process lines until #endif)
-  var STATE_ELSE_TRUE = 2;
+  const STATE_ELSE_TRUE = 2;
   // inside if, condition true (process lines until #else or #endif)
-  var STATE_IF_TRUE = 3;
+  const STATE_IF_TRUE = 3;
   // inside else or elif, #if/#elif was true, so following #else or #elif is
   // false (ignore lines until #endif)
-  var STATE_ELSE_FALSE = 4;
+  const STATE_ELSE_FALSE = 4;
 
-  var line;
-  var state = STATE_NONE;
-  var stack = [];
-  var control = /^(?:\/\/|<!--)\s*#(if|elif|else|endif|expand|include|error)\b(?:\s+(.*?)(?:-->)?$)?/;
-  var lineNumber = 0;
+  let line;
+  let state = STATE_NONE;
+  const stack = [];
+  const control = /^(?:\/\/|<!--)\s*#(if|elif|else|endif|expand|include|error)\b(?:\s+(.*?)(?:-->)?$)?/;
+  let lineNumber = 0;
   var loc = function () {
     return fs.realpathSync(inFilename) + ":" + lineNumber;
   };
   while ((line = readLine()) !== null) {
     ++lineNumber;
-    var m = control.exec(line);
+    const m = control.exec(line);
     if (m) {
       switch (m[1]) {
         case "if":
@@ -205,27 +205,27 @@ function preprocessCSS(mode, source, destination) {
     return content.replace(
       /^\s*@import\s+url\(([^)]+)\);\s*$/gm,
       function (all, url) {
-        var file = path.join(path.dirname(baseUrl), url);
-        var imported = fs.readFileSync(file, "utf8").toString();
+        const file = path.join(path.dirname(baseUrl), url);
+        const imported = fs.readFileSync(file, "utf8").toString();
         return expandImports(imported, file);
       }
     );
   }
 
   function removePrefixed(content, hasPrefixedFilter) {
-    var lines = content.split(/\r?\n/g);
-    var i = 0;
+    const lines = content.split(/\r?\n/g);
+    let i = 0;
     while (i < lines.length) {
-      var line = lines[i];
+      const line = lines[i];
       if (!hasPrefixedFilter(line)) {
         i++;
         continue;
       }
       if (/\{\s*$/.test(line)) {
-        var bracketLevel = 1;
-        var j = i + 1;
+        let bracketLevel = 1;
+        let j = i + 1;
         while (j < lines.length && bracketLevel > 0) {
-          var checkBracket = /([{}])\s*$/.exec(lines[j]);
+          const checkBracket = /([{}])\s*$/.exec(lines[j]);
           if (checkBracket) {
             if (checkBracket[1] === "{") {
               bracketLevel++;
@@ -263,7 +263,7 @@ function preprocessCSS(mode, source, destination) {
     throw new Error("Invalid CSS preprocessor mode");
   }
 
-  var content = fs.readFileSync(source, "utf8").toString();
+  let content = fs.readFileSync(source, "utf8").toString();
   content = expandImports(content, source);
   if (mode === "mozcentral") {
     content = removePrefixed(content, hasPrefixedMozcentral);
@@ -277,7 +277,7 @@ exports.preprocessCSS = preprocessCSS;
  * the first.
  */
 function merge(defaults, defines) {
-  var ret = {};
+  const ret = {};
   for (var key in defaults) {
     ret[key] = defaults[key];
   }