Support tag configuration.

README.md

 `test` provides a standard way of writing and running tests in Dart.
 
 * [Writing Tests](#writing-tests)
 * [Running Tests](#running-tests)
   * [Restricting Tests to Certain Platforms](#restricting-tests-to-certain-platforms)
   * [Platform Selector Syntax](#platform-selector-syntax)
   * [Running Tests on Dartium](#running-tests-on-dartium)
 * [Asynchronous Tests](#asynchronous-tests)
 * [Running Tests With Custom HTML](#running-tests-with-custom-html)
 * [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)
 * [Testing with `barback`](#testing-with-barback)
 * [Further Reading](#further-reading)
 
 ## Writing Tests
 
 Tests are specified using the top-level [`test()`][test] function, and test
 assertions are made using [`expect()`][expect]:
 
 [test]: http://www.dartdocs.org/documentation/test/latest/index.html#test/test@id_test
@@ skipping 455 matching lines @@
 Both the annotation and the parameter take a map. The map's keys are [platform
 selectors](#platform-selector-syntax) 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.
 
+### 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/doc/package_config.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.
+
+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
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 available to tests.
 
 [api]: http://www.dartdocs.org/documentation/test/latest/index.html
 
 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/doc/json_reporter.md