pkgs/test/README.md

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`test` provides a standard way of writing and running tests in Dart.

## Using package:test

* [Writing Tests](#writing-tests)
* [Running Tests](#running-tests)
  * [Sharding Tests](#sharding-tests)
  * [Test Concurrency](#test-concurrency)
  * [Shuffling Tests](#shuffling-tests)
  * [Selecting a Test Reporter](#selecting-a-test-reporter)
  * [Collecting Code Coverage](#collecting-code-coverage)
  * [Restricting Tests to Certain Platforms](#restricting-tests-to-certain-platforms)
  * [Platform Selectors](#platform-selectors)
  * [Compiler Selectors](#compiler-selectors)
  * [Running Tests on Node.js](#running-tests-on-nodejs)
* [Asynchronous Tests](#asynchronous-tests)
* [Running Tests With Custom HTML](#running-tests-with-custom-html)
  * [Providing a custom HTML template](#providing-a-custom-html-template)
* [Configuring Tests](#configuring-tests)
  * [Skipping Tests](#skipping-tests)
  * [Timeouts](#timeouts)
  * [Platform-Specific Configuration](#platform-specific-configuration)
  * [Whole-Package Configuration](#whole-package-configuration)
* [Tagging Tests](#tagging-tests)
* [Debugging](#debugging)
* [Browser/VM Hybrid Tests](#browservm-hybrid-tests)
* [Support for Other Packages](#support-for-other-packages)
  * [`build_runner`](#build_runner)
  * [`term_glyph`](#term_glyph)
* [Further Reading](#further-reading)

## Writing Tests

Tests are specified using the top-level [`test()`] function.
Test asserts can be made using [`expect` from `package:matcher`][expect]

[`test()`]: https://pub.dev/documentation/test/latest/test/test.html

[expect]: https://pub.dev/documentation/matcher/latest/expect/expect.html

```dart
import 'package:test/test.dart';

void main() {
  test('String.split() splits the string on the delimiter', () {
    var string = 'foo,bar,baz';
    expect(string.split(','), equals(['foo', 'bar', 'baz']));
  });

  test('String.trim() removes surrounding whitespace', () {
    var string = '  foo ';
    expect(string.trim(), equals('foo'));
  });
}
```

Tests can be grouped together using the [`group()`] function. Each group's
description is added to the beginning of its test's descriptions.

[`group()`]: https://pub.dev/documentation/test/latest/test/group.html

```dart
import 'package:test/test.dart';

void main() {
  group('String', () {
    test('.split() splits the string on the delimiter', () {
      var string = 'foo,bar,baz';
      expect(string.split(','), equals(['foo', 'bar', 'baz']));
    });

    test('.trim() removes surrounding whitespace', () {
      var string = '  foo ';
      expect(string.trim(), equals('foo'));
    });
  });

  group('int', () {
    test('.remainder() returns the remainder of division', () {
      expect(11.remainder(3), equals(2));
    });

    test('.toRadixString() returns a hex string', () {
      expect(11.toRadixString(16), equals('b'));
    });
  });
}
```

You can use the [`setUp()`] and [`tearDown()`] functions to share code between
tests. The `setUp()` callback will run before every test in a group or test
suite, and `tearDown()` will run after. `tearDown()` will run even if a test
fails, to ensure that it has a chance to clean up after itself.

```dart
import 'package:test/test.dart';

void main() {
  late HttpServer server;
  late Uri url;
  setUp(() async {
    server = await HttpServer.bind('localhost', 0);
    url = Uri.parse('http://${server.address.host}:${server.port}');
  });

  tearDown(() async {
    await server.close(force: true);
    server = null;
    url = null;
  });

  // ...
}
```

[`setUp()`]: https://pub.dev/documentation/test/latest/test/setUp.html

[`tearDown()`]: https://pub.dev/documentation/test/latest/test/tearDown.html

## Running Tests

A single test file can be run just using `dart test path/to/test.dart` (as of
Dart 2.10 - prior sdk versions must use `pub run test` instead of `dart test`).

![Single file being run via "dart test"](https://raw.githubusercontent.com/dart-lang/test/master/pkgs/test/image/test1.gif)

Many tests can be run at a time using `dart test path/to/dir`.

![Directory being run via "dart test".](https://raw.githubusercontent.com/dart-lang/test/master/pkgs/test/image/test2.gif)

It's also possible to run a test on the Dart VM only by invoking it using `dart
path/to/test.dart`, but this doesn't load the full test runner and will be
missing some features.

The test runner accepts one or more path arguments. If there are no path
arguments the runner defaults to running tests under the `test/` directory. When
running for `test/` or any other directory, the runner will recursively search
the directory for files that match the test name pattern `*_test.dart`. The
pattern can be overridden in `dart_test.yaml`. When a path argument is a file
instead of a directory it will be run as a test, regardless of the file name.
Arguments which use shell globbing should avoid including non-test files in the
path argument. Tests which are run by other test runners may use a different
default path, such as `integration_test/`. Directories other than `test/` are
ignored by `dart test` unless passed explicitly as a path to run.

You can select specific tests cases to run by name using `dart test -n "test
name"`. The string is interpreted as a regular expression, and only tests whose
description (including any group descriptions) match that regular expression
will be run. You can also use the `-N` flag to run tests whose names contain a
plain-text string.

By default, tests are run in the Dart VM, but you can run them in the browser as
well by passing `dart test -p chrome path/to/test.dart`. `test` will take
care of starting the browser and loading the tests, and all the results will be
reported on the command line just like for VM tests. In fact, you can even run
tests on both platforms with a single command: `dart test -p "chrome,vm"
path/to/test.dart`.

By default each platform has a default compiler, but some of them support
more than one compiler. You can choose which compiler to use by passing
`dart test -c source`, which would run all VM tests from source instead of
compiling them to kernel. This also supports targeting a specific platform
using normal platform selectors, like this `dart test -c vm:source`.

### Test Path Queries

Some query parameters are supported on test paths, which allow you to filter the
tests that will run within just those paths. These filters are merged with any
global options that are passed, and all filters must match for a test to be ran.

- **name**: Works the same as `--name` (simple contains check).
  - This is the only option that supports more than one entry.
- **full-name**: Requires an exact match for the name of the test.
- **line**: Matches any test that originates from this line in the test suite.
- **col**: Matches any test that originates from this column in the test suite.

**Example Usage**: `dart test "path/to/test.dart?line=10&col=2"`

#### Line/Col Matching Semantics

The `line` and `col` filters match against the current stack trace taken from
the invocation to the `test` function, and are considered a match if
**any frame** in the trace meets **all** of the following criteria:

* The URI of the frame matches the root test suite uri.
  * This means it will not match lines from imported libraries.
* If both `line` and `col` are passed, both must match **the same frame**.
* The specific `line` and `col` to be matched are defined by the tools creating
  the stack trace. This generally means they are 1 based and not 0 based, but
  this package is not in control of the exact semantics and they may vary based
  on platform implementations.

### Sharding Tests

Tests can also be sharded with the `--total-shards` and `--shard-index` arguments,
allowing you to split up your test suites and run them separately. For example,
if you wanted to run 3 shards of your test suite, you could run them as follows:

```bash
dart test --total-shards 3 --shard-index 0 path/to/test.dart
dart test --total-shards 3 --shard-index 1 path/to/test.dart
dart test --total-shards 3 --shard-index 2 path/to/test.dart
```
Sharding: This refers to the process of splitting up a large test suite into
smaller subsets (called shards) that can be run independently. Sharding is
particularly useful for distributed testing, where multiple machines are used
to run tests simultaneously. By dividing the test suite into smaller subsets,
you can run tests in parallel across multiple machines, which can significantly
reduce the overall testing time.

### Test concurrency

Test suites run concurrently by default, using half of the host's CPU cores. Use
`--concurrency` to control the number of test suites that runs concurrently,
meaning that multiple tests in independent suites or platforms can run at the
same time. For example, if you wanted to run the tests on 4 threads, you could
run the tests as follows:

```bash
dart test --concurrency=4
```
This can speed up the overall testing process, especially if you have a large
number of test suites.

### Shuffling Tests

Test order can be shuffled with the `--test-randomize-ordering-seed` argument.
This allows you to shuffle your tests with a specific seed (deterministic) or
a random seed for each run. For example, consider the following test runs:

```bash
dart test --test-randomize-ordering-seed=12345
dart test --test-randomize-ordering-seed=random
```

Setting `--test-randomize-ordering-seed=0` will have the same effect as not
specifying it at all, meaning the test order will remain as-is.

### Selecting a Test Reporter

You can adjust the output format of test results using the `--reporter=<option>`
command line flag. The default format is the `compact` output format - a single
line, continuously updated as tests are run. When running on the GitHub Actions CI
however (detected via checking the `GITHUB_ACTIONS` environment variable for `true`),
the default changes to the `github` output format - a reporter customized
for that CI/CD system.

The available options for the `--reporter` flag are:

- `compact`: a single, continuously updated line
- `expanded`: a separate line for each update
- `github`: a custom reporter for GitHub Actions
- `json`: a machine-readable format; see https://dart.dev/go/test-docs/json_reporter.md

### Collecting Code Coverage

To collect code coverage, you can run tests with the `--coverage-path <file>`
argument, which outputs a LCOV report to the given file path. The file
specified can be an absolute or relative path. If a directory does not exist at
the path specified, a directory will be created. If a directory does exist,
files may be overwritten with the latest coverage data, if they conflict.

Coverage gathering is currently only implemented for tests run on the Dart VM or
Chrome.

Here's an example of how to run tests and collect coverage:

```shell
## Run Dart tests and output coverage info to `./coverage/lcov.info`:
dart run test --coverage-path=./coverage/lcov.info

## Generate a human readable report:
genhtml -o ./coverage/report ./coverage/lcov.info

## Open the coverage report:
open ./coverage/report/index.html
```

* *LCOV is a GNU tool which provides information about what parts of a program are
  actually executed (i.e. "covered") while running a particular test case.*
* The binary `genhtml` is one of the LCOV tools.
* See the LCOV project for more: https://github.com/linux-test-project/lcov
* See the Homebrew LCOV formula: https://formulae.brew.sh/formula/lcov

### Restricting Tests to Certain Platforms

Some test files only make sense to run on particular platforms. They may use
`dart:html` or `dart:io`, they might test Windows' particular filesystem
behavior, or they might use a feature that's only available in Chrome. The
[`@TestOn`] annotation makes it easy to declare exactly which platforms a test
file should run on. Just put it at the top of your file, before any `library` or
`import` declarations:

```dart
@TestOn('vm')

import 'dart:io';

import 'package:test/test.dart';

void main() {
  // ...
}
```

[`@TestOn`]: https://pub.dev/documentation/test/latest/test/TestOn-class.html

The string you pass to `@TestOn` is what's called a "platform selector", and it
specifies exactly which platforms a test can run on. It can be as simple as the
name of a platform, or a more complex Dart-like boolean expression involving
these platform names.

You can also declare that your entire package only works on certain platforms by
adding a [`test_on` field] to your package config file.

[`test_on` field]: https://github.com/dart-lang/test/blob/master/pkgs/test/doc/configuration.md#test_on

### Platform Selectors

Platform selectors use the [boolean selector syntax] defined in the
[`boolean_selector`] package, which is a subset of Dart's expression syntax that
only supports boolean operations. The platform selectors identify overlapping
subsets of the ways that Dart code can be compiled and run.

[boolean selector syntax]: https://github.com/dart-lang/boolean_selector/blob/master/README.md

[`boolean_selector`]: https://pub.dev/packages/boolean_selector

The following identifiers are defined:

* `vm`: Whether the test is running on the command-line Dart VM.

* `chrome`: Whether the test is running on Google Chrome.

* `firefox`: Whether the test is running on Mozilla Firefox.

* `safari`: Whether the test is running on Apple Safari.

* `edge`: Whether the test is running on Microsoft Edge browser.

* `node`: Whether the test is running on Node.js.

* `dart-vm`: Whether the test is running on the Dart VM in any context. It's
  identical to `!js`.

* `browser`: Whether the test is running in any browser.

* `js`: Whether the test has been compiled to JS. This is identical to
  `!dart-vm`.

* `blink`: Whether the test is running in a browser that uses the Blink
  rendering engine.

* `windows`: Whether the test is running on Windows. This can only be `true` if
  either `vm` or `node` is true.

* `mac-os`: Whether the test is running on MacOS. This can only be `true` if
  either `vm` or `node` is true.

* `linux`: Whether the test is running on Linux. This can only be `true` if
  either `vm` or `node` is true.

* `android`: Whether the test is running on Android. If `vm` is false, this will
  be `false` as well, which means that this *won't* be true if the test is
  running on an Android browser.

* `ios`: Whether the test is running on iOS. If `vm` is false, this will be
  `false` as well, which means that this *won't* be true if the test is running
  on an iOS browser.

* `posix`: Whether the test is running on a POSIX operating system. This is
  equivalent to `!windows`.

* `dart2js`: Whether the test has been compiled with Dart2Js.

* `dart2wasm`: Whether the test has been compiled with Dart2Wasm.

* `kernel`: Whether the test has been compiled to kernel.

* `source`: Whether the test has been run with no compiler (from source).

For example, if you wanted to run a test on every browser but Chrome compiled
with dart2js (in opposition to dart2wasm), you would write
`@TestOn('browser && !chrome && dart2js')`.

### Running Tests on Node.js

The test runner also supports compiling tests to JavaScript and running them on
[Node.js] by passing `--platform node`. Note that Node has access to *neither*
`dart:html` nor `dart:io`, so any platform-specific APIs will have to be invoked
using the [`js`] package. However, it may be useful when testing APIs that are
meant to be used by JavaScript code.

[Node.js]: https://nodejs.org/en/

[`js`]: https://pub.dev/packages/js

The test runner looks for an executable named `node` (on Mac OS or Linux) or
`node.exe` (on Windows) on your system path. When compiling Node.js tests, it
passes `-Dnode=true`, so tests can determine whether they're running on Node
using [`const bool.fromEnvironment('node')`][bool.fromEnvironment]. It also sets
`--server-mode`, which will tell the compiler that `dart:html` is not available.

[bool.fromEnvironment]: https://api.dart.dev/stable/dart-core/bool/bool.fromEnvironment.html

If a top-level `node_modules` directory exists, tests running on Node.js can
import modules from it.

## Asynchronous Tests

Tests written with `async`/`await` will work automatically. The test runner
won't consider the test finished until the returned `Future` completes.

```dart
import 'dart:async';

import 'package:test/test.dart';

void main() {
  test('Future.value() returns the value', () async {
    var value = await Future.value(10);
    expect(value, equals(10));
  });
}
```

### Uncaught Async Errors

Any uncaught asynchronous error throws within the zone that a test is running in
will cause the test to be considered a failure. This can cause a test which was
previously considered complete and passing to change into a failure if the
uncaught async error is raised late. If all test cases within the suite have
completed this may cause some errors to be missed, or to surface in only some
runs.

Avoid uncaught async errors by ensuring that all futures have an error handler
[before they complete as an error][early-handler].

[early-handler]:https://dart.dev/guides/libraries/futures-error-handling#potential-problem-failing-to-register-error-handlers-early

## Running Tests With Custom HTML

By default, the test runner will generate its own empty HTML file for browser
tests. However, tests that need custom HTML can create their own files. These
files have three requirements:

* They must have the same name as the test, with `.dart` replaced by `.html`. You can also
  provide a configuration path to an HTML file if you want it to be reused across all tests.
  See [Providing a custom HTML template](#providing-a-custom-html-template) below.

* They must contain a `link` tag with `rel="x-dart-test"` and an `href`
  attribute pointing to the test script.

* They must contain `<script src="packages/test/dart.js"></script>`.

For example, if you had a test called `custom_html_test.dart`, you might write
the following HTML file:

```html
<!doctype html>
<!-- custom_html_test.html -->
<html>
  <head>
    <title>Custom HTML Test</title>
    <link rel="x-dart-test" href="custom_html_test.dart">
    <script src="packages/test/dart.js"></script>
  </head>
  <body>
    // ...
  </body>
</html>
```

### Providing a custom HTML template

If you want to share the same HTML file across all tests, you can provide a
`custom_html_template_path` configuration option to your configuration file.
This file should follow the rules above, except that instead of the link tag
add exactly one `{{testScript}}` in the place where you want the template processor to insert it.

You can also optionally use any number of `{{testName}}` placeholders which will be replaced by the test filename.

The template can't be named like any test file, as that would clash with using the
custom HTML mechanics. In such a case, an error will be thrown.

For example:

```yaml
custom_html_template_path: html_template.html.tpl
```

```html
<!doctype html>
<!-- html_template.html.tpl -->
<html>
  <head>
    <title>{{testName}} Test</title>
    {{testScript}}
    <script src="packages/test/dart.js"></script>
  </head>
  <body>
    // ...
  </body>
</html>
```

## Configuring Tests

### Skipping Tests

If a test, group, or entire suite isn't working yet, and you just want it to stop
complaining, you can mark it as "skipped". The test or tests won't be run, and,
if you supply a reason why, that reason will be printed. In general, skipping
tests indicates that they should run but is temporarily not working. If they're
fundamentally incompatible with a platform, [`@TestOn`/`testOn`][TestOn]
should be used instead.

[TestOn]: #restricting-tests-to-certain-platforms

To skip a test suite, put a `@Skip` annotation at the top of the file:

```dart
@Skip('currently failing (see issue 1234)')

import 'package:test/test.dart';

void main() {
  // ...
}
```

The string you pass should describe why the test is skipped. You don't have to
include it, but it's a good idea to document why the test isn't running.

Groups and individual tests can be skipped by passing the `skip` parameter. This
can be either `true` or a String describing why the test is skipped. For
example:

```dart
import 'package:test/test.dart';

void main() {
  group('complicated algorithm tests', () {
    // ...
  }, skip: "the algorithm isn't quite right");

  test('error-checking test', () {
    // ...
  }, skip: 'TODO: add error-checking.');
}
```

### Timeouts

By default, tests will time out after 30 seconds of inactivity. The timeout
applies to deadlocks or cases where the test stops making progress, it does not
ensure that an overall test case or test suite completes within any set time.

Timeouts can be configured on a per-test, -group, or -suite basis. To change the
timeout for a test suite, put a `@Timeout` annotation at the top of the file:

```dart
@Timeout(Duration(seconds: 45))

import 'package:test/test.dart';

void main() {
  // ...
}
```

In addition to setting an absolute timeout, you can set the timeout relative to
the default using `@Timeout.factor`. For example, `@Timeout.factor(1.5)` will
set the timeout to one and a half times as long as the default—45 seconds.

Timeouts can be set for tests and groups using the `timeout` parameter. This
parameter takes a `Timeout` object just like the annotation. For example:

```dart
import 'package:test/test.dart';

void main() {
  group('slow tests', () {
    // ...

    test('even slower test', () {
      // ...
    }, timeout: Timeout.factor(2));
  }, timeout: Timeout(Duration(minutes: 1)));
}
```

Nested timeouts apply in order from outermost to innermost. That means that
"even slower test" will take two minutes to time out, since it multiplies the
group's timeout by 2.

### Platform-Specific Configuration

Sometimes a test may need to be configured differently for different platforms.
Windows might run your code slower than other platforms, or your DOM
manipulation might not work right on Safari yet. For these cases, you can use
the `@OnPlatform` annotation and the `onPlatform` named parameter to `test()`
and `group()`. For example:

```dart
@OnPlatform({
  // Give Windows some extra wiggle-room before timing out.
  'windows': Timeout.factor(2)
})

import 'package:test/test.dart';

void main() {
  test('do a thing', () {
    // ...
  }, onPlatform: {
    'safari': Skip('Safari is currently broken (see #1234)')
  });
}
```

Both the annotation and the parameter take a map. The map's keys are [platform
selectors](#platform-selectors) which describe the platforms for which the
specialized configuration applies. Its values are instances of some of the same
annotation classes that can be used for a suite: `Skip` and `Timeout`. A value
can also be a list of these values.

If multiple platforms match, the configuration is applied in order from first to
last, just as they would in nested groups. This means that for configuration
like duration-based timeouts, the last matching value wins.

You can also set up global platform-specific configuration using the
[package configuration file][configuring platforms].

[configuring platforms]: https://github.com/dart-lang/test/blob/master/pkgs/test/doc/configuration.md#configuring-platforms

### Tagging Tests

Tags are short strings that you can associate with tests, groups, and suites.
They don't have any built-in meaning, but they're very useful nonetheless: you
can associate your own custom configuration with them, or you can use them to
easily filter tests so you only run the ones you need to.

Tags are defined using the `@Tags` annotation for suites and the `tags` named
parameter to `test()` and `group()`. For example:

```dart
@Tags(['browser'])

import 'package:test/test.dart';

void main() {
  test('successfully launches Chrome', () {
    // ...
  }, tags: 'chrome');

  test('launches two browsers at once', () {
    // ...
  }, tags: ['chrome', 'firefox']);
}
```

If the test runner encounters a tag that wasn't declared in the
[package configuration file][configuring tags], it'll print a warning, so be
sure to include all your tags there. You can also use the file to provide
default configuration for tags, like giving all `browser` tests twice as much
time before they time out.

[configuring tags]: https://github.com/dart-lang/test/blob/master/pkgs/test/doc/configuration.md#configuring-tags

Tests can be filtered based on their tags by passing command line flags. The
`--tags` or `-t` flag will cause the test runner to only run tests with the
given tags, and the `--exclude-tags` or `-x` flag will cause it to only run
tests *without* the given tags. These flags also support
[boolean selector syntax]. For example, you can pass `--tags "(chrome ||
firefox) && !slow"` to select quick Chrome or Firefox tests.

Note that tags must be valid Dart identifiers, although they may also contain
hyphens.

### Whole-Package Configuration

For configuration that applies across multiple files, or even the entire
package, `test` supports a configuration file called `dart_test.yaml`. At its
simplest, this file can contain the same sort of configuration that can be
passed as command-line arguments:

```yaml
# This package's tests are very slow. Double the default timeout.
timeout: 2x

# This is a browser-only package, so test on chrome by default.
platforms: [chrome]
```

The configuration file sets new defaults. These defaults can still be overridden
by command-line arguments, just like the built-in defaults. In the example
above, you could pass `--platform firefox` to run on Firefox.

A configuration file can do much more than just set global defaults. See
[the full documentation][package config] for more details.

[package config]: https://github.com/dart-lang/test/blob/master/pkgs/test/doc/configuration.md

### Compiler flags

The test runner does not support general purpose flags to control compilation
such as `-D` defines or flags like `--no-sound-null-safety`. In most cases it is
preferable to avoid writing tests that depend on the fine-grained compiler
configuration. For instance to choose between sound and unsound null safety,
prefer to choose a language version for each test which has the desired behavior
by default - choose a language version below `2.12` to disable sound null
safety, and a language version above `2.12` to enable sound null safety. When
fine-grained configuration is unavoidable, the approach varies by platform.

Compilation for browser and node tests can be configured by passing arguments to
`dart compile js` with `--dart2js-args` options.

Fine-grained compilation configuration is not supported for the VM. Any
configuration which impacts runtime behavior for the entire VM, such as `-D`
defines (when used for non-const values) and runtime behavior experiments, will
influence both the test runner and the isolates spawned to run test suites.
Experiments which are breaking may cause incompatibilities with the test runner.
These may be specified with a `DART_VM_OPTIONS` environment variable when
running with `pub run test`, or by passing them to the `dart` command before the
`test` subcommand when using `dart test`.

## Debugging

Tests can be debugged interactively using platforms' built-in development tools.
Tests running on browsers can use those browsers' development consoles to inspect
the document, set breakpoints, and step through code. Those running on the Dart
VM use [Dart DevTools][devtools].

[devtools]: https://dart.dev/tools/dart-devtools

The first step when debugging is to pass the `--pause-after-load` flag to the
test runner. This pauses the browser after each test suite has loaded, so that
you have time to open the development tools and set breakpoints. For the Dart VM
it will print the remote debugger URL.

Once you've set breakpoints, either click the big arrow in the middle of the web
page or press Enter in your terminal to start the tests running. When you hit a
breakpoint, the runner will open its own debugging console in the terminal that
controls how tests are run. You can type "restart" there to re-run your test as
many times as you need to figure out what's going on.

Normally, browser tests are run in hidden iframes. However, when debugging, the
iframe for the current test suite is expanded to fill the browser window so you
can see and interact with any HTML it renders. Note that the Dart animation may
still be visible behind the iframe; to hide it, just add a `background-color` to
the page's HTML.

## Browser/VM Hybrid Tests

Code that's written for the browser often needs to talk to some kind of server.
Maybe you're testing the HTML served by your app, or maybe you're writing a
library that communicates over WebSockets. We call tests that run code on both
the browser and the VM **hybrid tests**.

Hybrid tests use one of two functions: [`spawnHybridCode()`] and
[`spawnHybridUri()`]. Both of these spawn Dart VM
[isolates][dart:isolate] that can import `dart:io` and other VM-only libraries.
The only difference is where the code from the isolate comes from:
`spawnHybridCode()` takes a chunk of actual Dart code, whereas
`spawnHybridUri()` takes a URL. They both return a [`StreamChannel`] that
communicates with the hybrid isolate. For example:

[`spawnHybridCode()`]: https://pub.dev/documentation/test/latest/test/spawnHybridCode.html

[`spawnHybridUri()`]: https://pub.dev/documentation/test/latest/test/spawnHybridUri.html

[dart:isolate]: https://api.dart.dev/stable/dart-isolate/dart-isolate-library.html

[`StreamChannel`]: https://pub.dev/documentation/stream_channel/latest/stream_channel/StreamChannel-class.html

```dart
// ## test/web_socket_server.dart

// The library loaded by spawnHybridUri() can import any packages that your
// package depends on, including those that only work on the VM.
import 'package:shelf/shelf_io.dart' as io;
import 'package:shelf_web_socket/shelf_web_socket.dart';
import 'package:stream_channel/stream_channel.dart';

// Once the hybrid isolate starts, it will call the special function
// hybridMain() with a StreamChannel that's connected to the channel
// returned spawnHybridCode().
hybridMain(StreamChannel channel) async {
  // Start a WebSocket server that just sends "hello!" to its clients.
  var server = await io.serve(webSocketHandler((webSocket, _) {
    webSocket.sink.add('hello!');
  }), 'localhost', 0);

  // Send the port number of the WebSocket server to the browser test, so
  // it knows what to connect to.
  channel.sink.add(server.port);
}


// ## test/web_socket_test.dart

@TestOn('browser')

import 'dart:html';

import 'package:test/test.dart';

void main() {
  test('connects to a server-side WebSocket', () async {
    // Each spawnHybrid function returns a StreamChannel that communicates with
    // the hybrid isolate. You can close this channel to kill the isolate.
    var channel = spawnHybridUri('web_socket_server.dart');

    // Get the port for the WebSocket server from the hybrid isolate.
    var port = await channel.stream.first;

    var socket = WebSocket('ws://localhost:$port');
    var message = await socket.onMessage.first;
    expect(message.data, equals('hello!'));
  });
}
```

![A diagram showing a test in a browser communicating with a Dart VM isolate outside the browser.](https://raw.githubusercontent.com/dart-lang/test/master/pkgs/test/image/hybrid.png)

**Note**: If you write hybrid tests, be sure to add a dependency on the
`stream_channel` package, since you're using its API!

## Support for Other Packages

### `build_runner`

If you are using `package:build_runner` to build your package, then you will
need a dependency on `build_test` in your `dev_dependencies`, and then you can
use the `pub run build_runner test` command to run tests.

To supply arguments to `package:test`, you need to separate them from your build
args with a `--` argument. For example, running all web tests in release mode
would look like this `pub run build_runner test --release -- -p vm`.

### `term_glyph`

The [`term_glyph`] package provides getters for Unicode glyphs with
ASCII alternatives. `test` ensures that it's configured to produce ASCII when
the user is running on Windows, where Unicode isn't supported. This ensures that
testing libraries can use Unicode on POSIX operating systems without breaking
Windows users.

[`term_glyph`]: https://pub.dev/packages/term_glyph

## Further Reading

Check out the [API docs] for detailed information about all the functions
available to tests.

[API docs]: https://pub.dev/documentation/test/latest/

The test runner also supports a machine-readable JSON-based reporter. This
reporter allows the test runner to be wrapped and its progress presented in
custom ways (for example, in an IDE). See [the protocol documentation][json] for
more details.

[json]: https://github.com/dart-lang/test/blob/master/pkgs/test/doc/json_reporter.md