As per appveyor/ci#3502.

trivial typo

New script will take care about building assets and also checking that bundled assets are up-to-date as part of CI.

This provides some improvements over the previous approach from f5521df#commitcomment-38967493:

1. It is possible to install `karma` from GitHub branch without the assumption that `browserify` is installed in user's project.
1. Karma contributors no longer need to run `npm run build` unless they want to change client code.
1. Simplifies runtime code.

Add a delay in execute to ensure that reload events and clear context events are completed first.
Fixes #3424

Currently it seems that it's not possible for reporters to set the exit
code asynchronously.
Within the "onRunComplete" event, the results.exitCode must be modified
synchronously, otherwise the updated value is not taken into account.

With this change, the reporters (or any other plugin) can pass an exit
code to the callback of the 'exit' event.

By locking sinon to the older version. The problem was that new version of sinon brought newer versions of several dependencies, which use ES2016 Map, which is not available in the older IE versions. The solution is fragile as we can't reliably lock transitive dependencies, but should do for now to make master green.

As a proper solution we should consider cutting a new major release, where we would drop support for ancient browsers and more clearly document which browsers karma aims to support.

# [5.2.0](v5.1.1...v5.2.0) (2020-08-31)

### Bug Fixes

* **client:** avoid race between execute and clearContext ([#3452](#3452)) ([8bc5b46](8bc5b46)), closes [#3424](#3424)
* **client:** check in bundled client code into version control ([#3524](#3524)) ([6cd5a3b](6cd5a3b)), closes [/github.com/karma-runner/karma/commit/f5521df7df5cd1201b5dce28dc4e326b1ffc41fd#commitcomment-38967493](https://github.com//github.com/karma-runner/karma/commit/f5521df7df5cd1201b5dce28dc4e326b1ffc41fd/issues/commitcomment-38967493)
* **dependencies:** update dependencies ([#3543](#3543)) ([5db46b7](5db46b7))
* **docs:** Update 03-how-it-works.md ([#3539](#3539)) ([e7cf7b1](e7cf7b1))
* **server:** log error when file loading or preprocessing fails ([#3540](#3540)) ([fc2fd61](fc2fd61))

### Features

* **server:** allow 'exit' listeners to set exit code ([#3541](#3541)) ([7a94d33](7a94d33))