Add support for configuration presets.

CHANGELOG.md

 ## 0.12.12
 
+* Add support for [test presets][]. These are defined using the `presets` field
+  in the package configuration file. They can be selected by passing `--preset`
+  or `-P`, or by using the `add_presets` field in the package configuration
+  file.
+
 * Add an `on_os` field to the package configuration file that allows users to
   select different configuration for different operating systems.
 
 * Add an `on_platform` field to the package configuration file that allows users
   to configure all tests differently depending on which platform they run on.
 
 * Add an `ios` platform selector variable. This variable will only be true when
   the `test` executable itself is running on iOS, not when it's running browser
   tests on an iOS browser.
 
+[test presets]: https://github.com/dart-lang/test/blob/master/doc/package_config.md#configuration-presets
+
 ## 0.12.11+2
 
 * Update to `shelf_web_socket` 0.2.0.
 
 ## 0.12.11+1
 
 * Purely internal change.
 
 ## 0.12.11
 
@@ skipping 409 matching lines @@
 * **NEW!** `expectAsyncUntil` replaces the now deprecated `expectAsyncUntil0`,
     `expectAsyncUntil1` and `expectAsyncUntil2`
 * `TestCase`:
     * Removed properties: `setUp`, `tearDown`, `testFunction`
     * `enabled` is now get-only
     * Removed methods: `pass`, `fail`, `error`
 * `interactive_html_config.dart` has been removed.
 * `runTests`, `tearDown`, `setUp`, `test`, `group`, `solo_test`, and
   `solo_group` now throw a `StateError` if called while tests are running.
 * `rerunTests` has been removed.