Add support for a global configuration file.

CHANGELOG.md

 ## 0.12.13
 
+* Add support for a global configuration file. On Windows, this file defaults to
+  `%LOCALAPPDATA%\DartTest.yaml`. On Unix, it defaults to `~/.dart_test.yaml`.
+  It can also be explicitly set using the `DART_TEST_CONFIG` environment
+  variable. See [the configuration documentation][global config] for details.
+
 * The `--name` and `--plain-name` arguments may be passed more than once, and
   may be passed together. A test must match all name constraints in order to be
   run.
 
 * Add `names` and `plain_names` fields to the package configuration file. These
   allow presets to control which tests are run based on their names.
 
 * Add `include_tags` and `exclude_tags` fields to the package configuration
   file. These allow presets to control which tests are run based on their tags.
 
 * Add a `pause_after_load` field to the package configuration file. This allows
   presets to enable debugging mode.
 
+[global config]: https://github.com/dart-lang/test/blob/master/doc/configuration.md#global-configuration
+
 ## 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.
 
@@ skipping 428 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.