addTearDown function
Registers a function to be run after the current test.
This is called within a running test, and adds a tear-down only for the current test. It allows testing libraries to add cleanup logic as soon as there's something to clean up.
The callback is run before any callbacks registered with tearDown. Like
tearDown, the most recently registered callback is run first.
If this is called from within a setUpAll or tearDownAll callback, it instead runs the function after all tests in the current test suite.
Implementation
void addTearDown(callback()) {
if (Invoker.current == null) {
throw StateError("addTearDown() may only be called within a test.");
}
Invoker.current.addTearDown(callback);
}