revert rollup 2 upgrade due to issues w/worker chunk in UMD build

This reverts commit c7a9871.

karma.conf.js

@@ -1,10 +1,12 @@
-const babel = require('@rollup/plugin-babel').default;
-const resolve = require('@rollup/plugin-node-resolve').default;
-const commonjs = require('@rollup/plugin-commonjs');
-const globals = require('rollup-plugin-node-globals');
-const builtins = require('rollup-plugin-node-builtins');
-const json = require('@rollup/plugin-json');
-const string = require('rollup-plugin-string').string;
+/*jshint node: true*/
+
+var babel = require('rollup-plugin-babel');
+var resolve = require('rollup-plugin-node-resolve');
+var commonjs = require('rollup-plugin-commonjs');
+var globals = require('rollup-plugin-node-globals');
+var builtins = require('rollup-plugin-node-builtins');
+var json = require('rollup-plugin-json');
+var string = require('rollup-plugin-string');
 
 module.exports = function (config) {
 
@@ -46,7 +48,14 @@ module.exports = function (config) {
                     browser: true,
                     preferBuiltins: false
                 }),
-                commonjs(),
+                commonjs({
+                    // There hints are required for importing jszip
+                    // See https://rollupjs.org/guide/en#error-name-is-not-exported-by-module-
+                    namedExports: {
+                        'node_modules/process/browser.js': ['nextTick'],
+                        'node_modules/events/events.js': ['EventEmitter']
+                    }
+                }),
 
                 json({
                     exclude: ['node_modules/**', 'src/**'] // import JSON files