Refactored AxiosError to a constructor;

Refactored `Cancel` to a constructor, a subclass of the `AxiosError`;
Expose CanceledError class;
Refactored axios error codes;
Added `toFlatObject` util;

lib/cancel/Cancel.js

@@ -1,19 +0,0 @@
-'use strict';
-
-/**
- * A `Cancel` is an object that is thrown when an operation is canceled.
- *
- * @class
- * @param {string=} message The message.
- */
-function Cancel(message) {
-  this.message = message;
-}
-
-Cancel.prototype.toString = function toString() {
-  return 'Cancel' + (this.message ? ': ' + this.message : '');
-};
-
-Cancel.prototype.__CANCEL__ = true;
-
-module.exports = Cancel;

lib/cancel/CancelToken.js

@@ -1,6 +1,6 @@
 'use strict';
 
-var Cancel = require('./Cancel');
+var CanceledError = require('./CanceledError');
 
 /**
  * A `CancelToken` is an object that can be used to request cancellation of an operation.
@@ -56,13 +56,13 @@ function CancelToken(executor) {
       return;
     }
 
-    token.reason = new Cancel(message);
+    token.reason = new CanceledError(message);
     resolvePromise(token.reason);
   });
 }
 
 /**
- * Throws a `Cancel` if cancellation has been requested.
+ * Throws a `CanceledError` if cancellation has been requested.
  */
 CancelToken.prototype.throwIfRequested = function throwIfRequested() {
   if (this.reason) {

lib/cancel/CanceledError.js

@@ -0,0 +1,22 @@
+'use strict';
+
+var AxiosError = require('../core/AxiosError');
+var utils = require('../utils');
+
+/**
+ * A `CanceledError` is an object that is thrown when an operation is canceled.
+ *
+ * @class
+ * @param {string=} message The message.
+ */
+function CanceledError(message) {
+  // eslint-disable-next-line no-eq-null,eqeqeq
+  AxiosError.call(this, message == null ? 'canceled' : message, AxiosError.ERR_CANCELED);
+  this.name = 'CanceledError';
+}
+
+utils.inherits(CanceledError, AxiosError, {
+  __CANCEL__: true
+});
+
+module.exports = CanceledError;