lib: remove let from for loops

This is a known de-opt. It may not be 100% necessary in all cases but it
seems like a decent enough idea to avoid it.

PR-URL: nodejs#8873
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

lib/_stream_readable.js

@@ -661,14 +661,14 @@ Readable.prototype.unpipe = function(dest) {
     state.pipesCount = 0;
     state.flowing = false;
 
-    for (let i = 0; i < len; i++)
+    for (var i = 0; i < len; i++)
       dests[i].emit('unpipe', this);
     return this;
   }
 
   // try to find the right one.
-  const i = state.pipes.indexOf(dest);
-  if (i === -1)
+  const index = state.pipes.indexOf(dest);
+  if (index === -1)
     return this;
 
   state.pipes.splice(i, 1);

lib/internal/readline.js

@@ -26,7 +26,7 @@ function getStringWidth(str) {
 
   str = stripVTControlCharacters(str);
 
-  for (let i = 0; i < str.length; i++) {
+  for (var i = 0; i < str.length; i++) {
     const code = str.codePointAt(i);
 
     if (code >= 0x10000) { // surrogates

lib/internal/util.js

@@ -24,7 +24,7 @@ exports.error = function(msg) {
   if (arguments.length > 1) {
     const args = new Array(arguments.length);
     args[0] = fmt;
-    for (let i = 1; i < arguments.length; ++i)
+    for (var i = 1; i < arguments.length; ++i)
       args[i] = arguments[i];
     console.error.apply(console, args);
   } else {

lib/punycode.js

@@ -210,7 +210,7 @@ const decode = function(input) {
 		basic = 0;
 	}
 
-	for (let j = 0; j < basic; ++j) {
+	for (var j = 0; j < basic; ++j) {
 		// if it's not a basic code point
 		if (input.charCodeAt(j) >= 0x80) {
 			error('not-basic');
@@ -221,15 +221,15 @@ const decode = function(input) {
 	// Main decoding loop: start just after the last delimiter if any basic code
 	// points were copied; start at the beginning otherwise.
 
-	for (let index = basic > 0 ? basic + 1 : 0; index < inputLength; /* no final expression */) {
+	for (var index = basic > 0 ? basic + 1 : 0; index < inputLength; /* no final expression */) {
 
 		// `index` is the index of the next character to be consumed.
 		// Decode a generalized variable-length integer into `delta`,
 		// which gets added to `i`. The overflow checking is easier
 		// if we increase `i` as we go, then subtract off its starting
 		// value at the end to obtain `delta`.
 		let oldi = i;
-		for (let w = 1, k = base; /* no condition */; k += base) {
+		for (var w = 1, k = base; /* no condition */; k += base) {
 
 			if (index >= inputLength) {
 				error('invalid-input');
@@ -345,7 +345,7 @@ const encode = function(input) {
 			if (currentValue == n) {
 				// Represent delta as a generalized variable-length integer.
 				let q = delta;
-				for (let k = base; /* no condition */; k += base) {
+				for (var k = base; /* no condition */; k += base) {
 					const t = k <= bias ? tMin : (k >= bias + tMax ? tMax : k - bias);
 					if (q < t) {
 						break;

lib/repl.js

@@ -345,7 +345,7 @@ function REPLServer(prompt,
 
     // After executing the current expression, store the values of RegExp
     // predefined properties back in `savedRegExMatches`
-    for (let idx = 1; idx < savedRegExMatches.length; idx += 1) {
+    for (var idx = 1; idx < savedRegExMatches.length; idx += 1) {
       savedRegExMatches[idx] = RegExp[`$${idx}`];
     }
 
@@ -1078,9 +1078,9 @@ function longestCommonPrefix(arr = []) {
 
   const first = arr[0];
   // complexity: O(m * n)
-  for (let m = 0; m < first.length; m++) {
+  for (var m = 0; m < first.length; m++) {
     const c = first[m];
-    for (let n = 1; n < cnt; n++) {
+    for (var n = 1; n < cnt; n++) {
       const entry = arr[n];
       if (m >= entry.length || c !== entry[m]) {
         return first.substring(0, m);

lib/tls.js

@@ -100,7 +100,7 @@ function check(hostParts, pattern, wildcards) {
     return false;
 
   // Check host parts from right to left first.
-  for (let i = hostParts.length - 1; i > 0; i -= 1)
+  for (var i = hostParts.length - 1; i > 0; i -= 1)
     if (hostParts[i] !== patternParts[i])
       return false;
 

lib/util.js

@@ -30,7 +30,7 @@ if (typeof global.SIMD === 'object' && global.SIMD !== null) {
   const make = (extractLane, count) => {
     return (ctx, value, recurseTimes, visibleKeys, keys) => {
       const output = new Array(count);
-      for (let i = 0; i < count; i += 1)
+      for (var i = 0; i < count; i += 1)
         output[i] = formatPrimitive(ctx, extractLane(value, i));
       return output;
     };

lib/v8.js

@@ -60,7 +60,7 @@ exports.getHeapSpaceStatistics = function() {
   const buffer = heapSpaceStatisticsBuffer;
   v8binding.updateHeapSpaceStatisticsArrayBuffer();
 
-  for (let i = 0; i < kNumberOfHeapSpaces; i++) {
+  for (var i = 0; i < kNumberOfHeapSpaces; i++) {
     const propertyOffset = i * kHeapSpaceStatisticsPropertiesCount;
     heapSpaceStatistics[i] = {
       space_name: kHeapSpaces[i],