Import fetch-mock package from source to avoid core-js version mismat…

…ch errors

Currently fetch-mock ships a transpiled copy of its code for use in the browser. This requires
several Babel runtime and Core JS helpers and polyfills. However the code is transpiled using
an older version of Babel which uses core-js v2 but everything we now use depends on core-js v3.

wheresrhys/fetch-mock#419

.storybook/build-story.js

@@ -4,7 +4,11 @@ import { storiesOf } from '@storybook/react';
 import * as knobsAddon from '@storybook/addon-knobs';
 import { Helmet } from 'react-helmet';
 import path from 'path';
-import fetchMock from 'fetch-mock';
+
+// HACK: The browser bundle for Fetch Mock implicitly depends on core-js 2.x.
+// We no longer use core-js 2.x anywhere so importing the browser bundle will fail.
+// </~https://github.com/wheresrhys/fetch-mock/issues/419>
+import fetchMock from 'fetch-mock/src/client.js';
 
 const defaultKnobs = () => ({});
 

.storybook/webpack.config.js

@@ -8,7 +8,8 @@ const xEngine = require('../packages/x-engine/src/webpack');
 const CopyPlugin = require('copy-webpack-plugin');
 const WritePlugin = require('write-file-webpack-plugin');
 
-const excludePaths = [/node_modules/, /dist/];
+// HACK: we need to include fetch-mock from source so it must be transpiled
+const excludePaths = [/node_modules\/(?!(fetch-mock)\/)/, /dist/];
 
 const cssCopy = fs.readdirSync(path.resolve('components')).reduce((mains, component) => {
 	const componentPkg = path.resolve('components', component, 'package.json');