build UMD bundle and polyfill language features (#4366)

- use rollup's `umd` bundle instead of `iife`
    - consume Rollup config's `output` prop in `karma-rollup-plugin`
    - simplify `rollup.config.js` `output` prop
    - fix invalid `script` tag `src` attrib in `lib/browser/template.html` (& reformat)
    - use core-js polyfills
- remove old ESlint rule to disallow `Object.assign` (it's now allowed... but so is object rest/spread, which works in many  cases)
- fix travis script
- karma-related:
    - update Symbol test for IE11, as the polyfill is just a polyfill.
    - add a proper RequireJS integration test
    - remove the old `bundle/amd.spec.js` test
    - try to make `karma.conf.js` less intimidating
    - allow custom externals/globals in karma rollup plugin
    - `unexpected`, `unexpected-eventemitter`, and `unexpected-sinon` are now external (not bundled) and loaded via their globals
    - modified `test/browser-specific/setup.js` to use `require()` since we can do that now
    - bundle now has an inline source map since I couldn't figure out any other way to load it
    - removed duplication of work in the karma plugin; no call to `bundle.generate()` is needed. furthermore the `code` prop did not include the inline source maps. so we just write the file and read the result. could probably avoid reading the file by manually stitching the `code` and `map` props together, but I'm unsure how
    - loads `mocha.js.map` for debugging

.eslintrc.yml

@@ -15,11 +15,6 @@ rules:
   strict:
     - error
     - safe
-  # disallow Object.assign
-  no-restricted-properties:
-    - error
-    - object: 'Object'
-      property: 'assign'
 overrides:
   - files:
       - 'docs/js/**/*.js'

.travis.yml

@@ -49,7 +49,7 @@ jobs:
       node_js: '10'
       name: 'Node.js v10'
 
-    - script: npm start test.bundle test.browser
+    - script: npm start test.browser
       name: 'Browser'
       node_js: 12
       install: npm ci # we need the native modules here

browser-entry.js

@@ -210,10 +210,10 @@ Mocha.process = process;
  * Expose mocha.
  */
 
-global.Mocha = Mocha;
-global.mocha = mocha;
+mocha.Mocha = Mocha;
+mocha.mocha = mocha;
 
 // this allows test/acceptance/required-tokens.js to pass; thus,
 // you can now do `const describe = require('mocha').describe` in a
 // browser context (assuming browserification).  should fix #880
-module.exports = global;
+module.exports = Object.assign(mocha, global);