Previously, the `.property` assertion failed ungracefully if the target was `null` or `undefined`. This commit causes an `AssertionError` to be thrown instead so that the ssfi flag and custom error messages are respected.

The `ssfi` flag doesn't need to be passed to the `expectTypes`
util function because it can access it directly from the assertion
object.

fix: check target's type in `.property` assertion

refactor: `expectTypes` to access `ssfi` flag

Add a missing var keyword found by lgtm.com

Update browserify to the latest version 🚀

* Fixed most tests

* Fix date uniformity (UTC)

* Add comment

* Cleanup

* Change extra functions (WIP)

* Finished tests

* Add master chai.js back

* Uncomment failing tests

* Update master and tests

* Actual seconds, duh

* Refactor 'above'

* Update all methods

* More explicit error

* Documentation - articles consistency

* Custom message test in assert

* No parenthesis in should

* Custom message for isAbove

* fix error messages tests

* add tests

* add implementation

* performance tweaks

* add tests for SameValueZero

* drop weakmap support

* update docs

Contrary to spec, IE11 uses SameValue instead of SameValueZero
equality for Sets.

test: skip failing `.include` tests in IE11

Create CODEOWNERS

chai@4.1.0