refactor stat_graph_contributors_graph to es6 module syntax

app/assets/javascripts/graphs/graphs_bundle.js

-require('./stat_graph_contributors_graph');
 require('./stat_graph_contributors');

app/assets/javascripts/graphs/stat_graph_contributors.js

 /* eslint-disable func-names, space-before-function-paren, wrap-iife, no-var, one-var, camelcase, one-var-declaration-per-line, quotes, no-param-reassign, quote-props, comma-dangle, prefer-template, max-len, no-return-assign */
-/* global ContributorsGraph */
-/* global ContributorsAuthorGraph */
-/* global ContributorsMasterGraph */
+
+import { ContributorsGraph, ContributorsAuthorGraph, ContributorsMasterGraph } from './stat_graph_contributors_graph';
 import ContributorsStatGraphUtil from './stat_graph_contributors_util';
 
 /* global d3 */

app/assets/javascripts/graphs/stat_graph_contributors_graph.js

-/* eslint-disable func-names, space-before-function-paren, one-var, no-var, prefer-rest-params, max-len, no-restricted-syntax, vars-on-top, no-use-before-define, no-param-reassign, new-cap, no-underscore-dangle, wrap-iife, comma-dangle, no-return-assign, prefer-arrow-callback, quotes, prefer-template, newline-per-chained-call, no-else-return */
-/* global d3 */
-/* global ContributorsGraph */
+/* eslint-disable func-names, space-before-function-paren, no-var, prefer-rest-params, max-len, no-restricted-syntax, vars-on-top, no-use-before-define, no-param-reassign, new-cap, no-underscore-dangle, wrap-iife, comma-dangle, no-return-assign, prefer-arrow-callback, quotes, prefer-template, newline-per-chained-call, no-else-return, no-shadow */
 
-window.d3 = require('d3');
+import d3 from 'd3';
 
-(function() {
-  var bind = function(fn, me) { return function() { return fn.apply(me, arguments); }; },
-    extend = function(child, parent) { for (var key in parent) { if (hasProp.call(parent, key)) child[key] = parent[key]; } function ctor() { this.constructor = child; } ctor.prototype = parent.prototype; child.prototype = new ctor(); child.__super__ = parent.prototype; return child; },
-    hasProp = {}.hasOwnProperty;
+const bind = function(fn, me) { return function() { return fn.apply(me, arguments); }; };
+const extend = function(child, parent) { for (var key in parent) { if (hasProp.call(parent, key)) child[key] = parent[key]; } function ctor() { this.constructor = child; } ctor.prototype = parent.prototype; child.prototype = new ctor(); child.__super__ = parent.prototype; return child; };
+const hasProp = {}.hasOwnProperty;
 
-  this.ContributorsGraph = (function() {
+export const ContributorsGraph = (function() {
   function ContributorsGraph() {}
 
   ContributorsGraph.prototype.MARGIN = {
@@ -91,9 +88,9 @@ window.d3 = require('d3');
   };
 
   return ContributorsGraph;
-  })();
+})();
 
-  this.ContributorsMasterGraph = (function(superClass) {
+export const ContributorsMasterGraph = (function(superClass) {
   extend(ContributorsMasterGraph, superClass);
 
   function ContributorsMasterGraph(data1) {
@@ -195,9 +192,9 @@ window.d3 = require('d3');
   };
 
   return ContributorsMasterGraph;
-  })(ContributorsGraph);
+})(ContributorsGraph);
 
-  this.ContributorsAuthorGraph = (function(superClass) {
+export const ContributorsAuthorGraph = (function(superClass) {
   extend(ContributorsAuthorGraph, superClass);
 
   function ContributorsAuthorGraph(data1) {
@@ -272,5 +269,4 @@ window.d3 = require('d3');
   };
 
   return ContributorsAuthorGraph;
-  })(ContributorsGraph);
-}).call(window);
+})(ContributorsGraph);

spec/javascripts/graphs/stat_graph_contributors_graph_spec.js

-/* eslint-disable quotes, jasmine/no-suite-dupes, vars-on-top, no-var, max-len */
-/* global d3 */
-/* global ContributorsGraph */
-/* global ContributorsMasterGraph */
+/* eslint-disable quotes, jasmine/no-suite-dupes, vars-on-top, no-var */
 
-require('~/graphs/stat_graph_contributors_graph');
+import d3 from 'd3';
+import { ContributorsGraph, ContributorsMasterGraph } from '~/graphs/stat_graph_contributors_graph';
 
 describe("ContributorsGraph", function () {
   describe("#set_x_domain", function () {