Used to assert that one value is equal to null.
The value that should equal null.
The file name that the assert failed in. Should be left as default.
The file line that the assert failed in. Should be left as default.
If value is NOT null, will throw an AssertError.
// Will throw an exception like "AssertError@example.d(6): expected to be <null>." string z = "blah"; z.shouldBeNull();