Axios ES2017 (#4787)

* Added AxiosHeaders class;

* Fixed README.md href;

* Fixed a potential bug with headers normalization;

* Fixed a potential bug with headers normalization;
Refactored accessor building routine;
Refactored default transforms;
Removed `normalizeHeaderName` helper;

* Added `Content-Length` accessor;
Added missed `has` accessor to TS types;

* Added `AxiosTransformStream` class;
Added progress capturing ability for node.js environment;
Added `maxRate` option to limit the data rate in node.js environment;
Refactored event handled by `onUploadProgress` && `onDownloadProgress` listeners in browser environment;
Added progress & data rate tests for the http adapter;
Added response stream aborting test;
Added a manual progress capture test for the browser;
Updated TS types;
Added TS tests;
Refactored request abort logic for the http adapter;
Added ability to abort the response stream;

* Remove `stream/promises` & `timers/promises` modules usage in tests;

* Use `abortcontroller-polyfill`;

* Fixed AxiosTransformStream dead-lock in legacy node versions;
Fixed CancelError emitting in streams;

* Reworked AxiosTransformStream internal logic to optimize memory consumption;
Added throwing an error if the request stream was silently destroying (without error) Refers to #3966;

* Treat the destruction of the request stream as a cancellation of the request;
Fixed tests;

* Emit `progress` event in the next tick;

* Initial refactoring;

* Refactored Mocha tests to use ESM;

* Refactored Karma tests to use rollup preprocessor & ESM;
Replaced grunt with gulp;
Improved dev scripts;
Added Babel for rollup build;

* Added default commonjs package export for Node build;
Added automatic contributors list generator for package.json;

Co-authored-by: Jay <jasonsaayman@gmail.com>

lib/helpers/toFormData.js

@@ -1,8 +1,8 @@
 'use strict';
 
-var utils = require('../utils');
-var AxiosError = require('../core/AxiosError');
-var envFormData = require('../env/classes/FormData');
+import utils from '../utils.js';
+import AxiosError from '../core/AxiosError.js';
+import envFormData from '../env/classes/FormData.js';
 
 /**
  * Determines if the given thing is a array or js object.
@@ -55,7 +55,7 @@ function isFlatArray(arr) {
   return utils.isArray(arr) && !arr.some(isVisitable);
 }
 
-var predicates = utils.toFlatObject(utils, {}, null, function filter(prop) {
+const predicates = utils.toFlatObject(utils, {}, null, function filter(prop) {
   return /^is[A-Z]/.test(prop);
 });
 
@@ -111,13 +111,13 @@ function toFormData(obj, formData, options) {
     return !utils.isUndefined(source[option]);
   });
 
-  var metaTokens = options.metaTokens;
+  const metaTokens = options.metaTokens;
   // eslint-disable-next-line no-use-before-define
-  var visitor = options.visitor || defaultVisitor;
-  var dots = options.dots;
-  var indexes = options.indexes;
-  var _Blob = options.Blob || typeof Blob !== 'undefined' && Blob;
-  var useBlob = _Blob && isSpecCompliant(formData);
+  const visitor = options.visitor || defaultVisitor;
+  const dots = options.dots;
+  const indexes = options.indexes;
+  const _Blob = options.Blob || typeof Blob !== 'undefined' && Blob;
+  const useBlob = _Blob && isSpecCompliant(formData);
 
   if (!utils.isFunction(visitor)) {
     throw new TypeError('visitor must be a function');
@@ -152,7 +152,7 @@ function toFormData(obj, formData, options) {
    * @returns {boolean} return true to visit the each prop of the value recursively
    */
   function defaultVisitor(value, key, path) {
-    var arr = value;
+    let arr = value;
 
     if (value && !path && typeof value === 'object') {
       if (utils.endsWith(key, '{}')) {
@@ -187,12 +187,12 @@ function toFormData(obj, formData, options) {
     return false;
   }
 
-  var stack = [];
+  const stack = [];
 
-  var exposedHelpers = Object.assign(predicates, {
-    defaultVisitor: defaultVisitor,
-    convertValue: convertValue,
-    isVisitable: isVisitable
+  const exposedHelpers = Object.assign(predicates, {
+    defaultVisitor,
+    convertValue,
+    isVisitable
   });
 
   function build(value, path) {
@@ -205,7 +205,7 @@ function toFormData(obj, formData, options) {
     stack.push(value);
 
     utils.forEach(value, function each(el, key) {
-      var result = !utils.isUndefined(el) && visitor.call(
+      const result = !utils.isUndefined(el) && visitor.call(
         formData, el, utils.isString(key) ? key.trim() : key, path, exposedHelpers
       );
 
@@ -226,4 +226,4 @@ function toFormData(obj, formData, options) {
   return formData;
 }
 
-module.exports = toFormData;
+export default toFormData;