How to write multiple unittests in dart in multiple files?

There is a tool that does exactly that, Dart Test Runner. An excerpt from that page:

Dart Test Runner will automatically detect and run all the tests in your Dart project in the correct environment (VM or Browser).

It detects any test writen in a file suffixed with _test.dart where your test code is inside a main() function. It doesn't have any problem detecting and running unittest tests.

It's pretty easy to install it and run it. Just two commands:

$ pub global activate test_runner
$ pub global run test_runner

For more options, please check Dart Test Runner page.

It is common to separate tests into multiple files. I am including an example of how you can do that.

Imagine that you have 2 files with tests, foo_test.dart, bar_test.dart that contain tests for your program. foo_test.dart could look something like this:

library foo_test;

import 'package:unittest/unittest.dart';

void main() {
  test('foo test', () {
    expect("foo".length, equals(3));
  });
}

And bar_test.dart could look something like this:

library bar_test;

import 'package:unittest/unittest.dart';

void main() {
  test('bar test', () {
    expect("bar".length, equals(3));
  });
}

You could run either file, and the test contained in that file would execute.

The, I would create something like an all_tests.dart file that would import the tests from foo_test.dart and bar_test.dart. Here is what all_tests.dart could look like:

import 'foo_test.dart' as foo_test;
import 'bar_test.dart' as bar_test;

void main() {
  foo_test.main();
  bar_test.main();
}

If you executed all_tests.dart, both the tests from foo_test.dart and bar_test.dart would execute.

One thing to note: for all this to work, you need to declare foo_test.dart and bar_test.dart as libraries (see the first line of each file). Then, in all_tests.dart, you can use import syntax to fetch the contents of the declared libraries.

This is how I organize most of my tests.