Add support for a global configuration file.

doc/configuration.md

 Each package may include a configuration file that applies to the package as a
 whole. This file can be used to provide custom defaults for various options, to
 define configuration for multiple files, and more.
 
 The file is named `dart_test.yaml` and lives at the root of the package, next to
 the package's pubspec. Like the pubspec, it's a [YAML][] file. Here's an
 example:
 
 [YAML]: http://yaml.org/
 
@@ skipping 33 matching lines @@
   * [`reporter`](#reporter)
 * [Configuring Tags](#configuring-tags)
   * [`tags`](#tags)
   * [`add_tags`](#add_tags)
 * [Configuring Platforms](#configuring-platforms)
   * [`on_os`](#on_os)
   * [`on_platform`](#on_platform)
 * [Configuration Presets](#configuration-presets)
   * [`presets`](#presets)
   * [`add_preset`](#add_preset)
+* [Global Configuration](#global-configuration)
 
 ## Test Configuration
 
 There are two major categories of configuration field: "test" and "runner". Test
 configuration controls how individual tests run, while
 [runner configuration](#runner-configuration) controls the test runner as a
 whole. Both types of fields may be used at the top level of a configuration
 file. However, because different tests can have different test configuration,
 only test configuration fields may be used to [configure tags](#tags) or
 [platforms](#on_platform).
@@ skipping 50 matching lines @@
 whether the tests are skipped or a string indicating the reason they're skipped.
 
 [test]: https://www.dartdocs.org/documentation/test/0.12.10+2/test/test.html
 
 ```yaml
 tags:
   chrome:
     skip: "Our Chrome launcher is busted. See issue 1234."
 ```
 
+This field is not supported in the
+[global configuration file](#global-configuration).
+
 ### `test_on`
 
 This field declares which platforms a test supports. It takes a
 [platform selector][platform selectors] and only allows tests to run on
 platforms that match the selector. It's often used with
 [specific tags](#configuring-tags) to ensure that certain features will only be
 tested on supported platforms.
 
 [platform selectors]: https://github.com/dart-lang/test/blob/master/README.md#platform-selectors
 
 ```yaml
 tags:
   # Internet Explorer doesn't support promises yet.
   promises: {test_on: "browser && !ie"}
 ```
 
 The field can also be used at the top level of the configuration file to
 indicate that the entire package only supports a particular platform. If someone
 tries to run the tests on an unsupported platform, the runner will print a
 warning and skip that platform.
 
 ```yaml
 # This package uses dart:io.
 test_on: vm
 ```
 
+This field is not supported in the
+[global configuration file](#global-configuration).
+
 ## Runner Configuration
 
 Unlike [test configuration](#test-configuration), runner configuration affects
 the test runner as a whole rather than individual tests. It can only be used at
 the top level of the configuration file.
 
 ### `paths`
 
 This field indicates the default paths that the test runner should run. These
 paths are usually directories, although single filenames may be used as well.
 Paths must be relative, and they must be in URL format so that they're
 compatible across operating systems. This defaults to `[test]`.
 
 ```yaml
 paths: [dart/test]
 
 paths:
 - test/instantaneous
 - test/fast
 - test/middling
 ```
 
+This field is not supported in the
+[global configuration file](#global-configuration).
+
 ### `filename`
 
 This field indicates the filename pattern that the test runner uses to find test
 files in directories. All files in directories passed on the command line (or in
 directories in [`paths`](#paths), if none are passed) whose basenames match this
 pattern will be loaded and run as tests.
 
 This supports the full [glob syntax][]. However, since it's only compared
 against a path's basename, path separators aren't especially useful. It defaults
 to `"*_test.dart"`.
 
 ```yaml
 filename: "test_*.dart"
 ```
 
 [glob syntax]: https://github.com/dart-lang/glob#syntax
 
+This field is not supported in the
+[global configuration file](#global-configuration).
+
 ### `names`
 
 This field causes the runner to only run tests whose names match the given
 regular expressions. A test's name must match *all* regular expressions in
 `names`, as well as containing all strings in [`plain_names`](#plain_names), in
 order to be run.
 
 This is usually used in a [preset](#configuration-presets) to make it possible
 to quickly select a given set of tests.
 
 ```yaml
 presets:
   # Pass "-P chrome" to run only Chrome tests.
   chrome:
     names:
     - "^browser:"
     - "[Cc]hrome"
 ```
 
+This field is not supported in the
+[global configuration file](#global-configuration).
+
 ### `plain_names`
 
 This field causes the runner to only run tests whose names contain the given
 strings. A test's name must contain *all* strings in `plain_names`, as well as
 matching all regular expressions in [`names`](#names), in order to be run.
 
 This is usually used in a [preset](#configuration-presets) to make it possible
 to quickly select a given set of tests.
 
 ```yaml
 presets:
   # Pass "-P ie" to run only Internet Explorer tests.
   ie:
     plain_names:
     - "IE"
     - "Internet Explorer"
 ```
 
+This field is not supported in the
+[global configuration file](#global-configuration).
+
 ### `include_tags`
 
 This field causes the runner to only run tests whose tags match the given
 [boolean selector][]. If both `include_tags` and [`exclude_tags`](#exclude_tags)
 are used, the exclusions take precedence.
 
 [boolean selector]: https://github.com/dart-lang/boolean_selector/blob/master/README.md
 
 This is usually used in a [preset](#configuration-preset) to make it possible to
 quickly select a set of tests.
 
 ```yaml
 presets:
   # Pass "-P windowless" to run tests that don't open browser windows.
   windowless:
     include_tags: !browser || content-shell
 ```
 
+This field is not supported in the
+[global configuration file](#global-configuration).
+
 ### `exclude_tags`
 
 This field causes the runner not to run tests whose tags match the given
 [boolean selector][]. If both [`include_tags`](#include_tags) and `exclude_tags`
 are used, the exclusions take precedence.
 
 This is usually used in a [preset](#configuration-preset) to make it possible to
 quickly select a set of tests.
 
 ```yaml
 presets:
   # Pass "-P windowless" to run tests that don't open browser windows.
   windowless:
     exclude_tags: browser && !content-shell
 ```
 
+This field is not supported in the
+[global configuration file](#global-configuration).
+
 ### `platforms`
 
 This field indicates which platforms tests should run on by default. It allows
 the same platform identifiers that can be passed to `--platform`. If multiple
 platforms are included, the test runner will default to running tests on all of
 them. This defaults to `[vm]`.
 
 ```yaml
 platforms: [content_shell]
 
@@ skipping 46 matching lines @@
 ### `reporter`
 
 This field indicates the default reporter to use. It may be set to "compact",
 "expanded", or "json" (although why anyone would want to default to JSON is
 beyond me). It defaults to "expanded" on Windows and "compact" everywhere else.
 
 ```yaml
 reporter: expanded
 ```
 
+This field is not supported in the
+[global configuration file](#global-configuration).
+
 ## Configuring Tags
 
 ### `tags`
 
 The `tag` field can be used to apply [test configuration](#test-configuration)
 to all tests [with a given tag][tagging tests] or set of tags. It takes a map
 from tag selectors to configuration maps. These configuration maps are just like
 the top level of the configuration file, except that they may not contain
 [runner configuration](#runner-configuration).
 
@@ skipping 35 matching lines @@
 Tag configuration is applied at whatever level the tag appears—so if a group is
 tagged as `integration`, its timeout will take precedence over the suite's
 timeout but not any tests'. If the group itself had a timeout declared, the
 group's explicit timeout would take precedence over the tag.
 
 If multiple tags appear at the same level, and they have conflicting
 configurations, the test runner *does not guarantee* what order they'll be
 resolved in. In practice, conflicting configuration is pretty unlikely and it's
 easy to just explicitly specify what you want on the test itself.
 
-This field counts as [test configuration](#test-configuration).
+This field counts as [test configuration](#test-configuration). It is not
+supported in the [global configuration file](#global-configuration).
 
 ### `add_tags`
 
 This field adds additional tags. It's technically
 [test configuration](#test-configuration), but it's usually used in more
 specific contexts. For example, when included in a tag's configuration, it can
 be used to enable tag inheritance, where adding one tag implicitly adds another
 as well. It takes a list of tag name strings.
 
 ```yaml
 tags:
   # Any test that spawns a browser.
   browser:
     timeout: 2x
 
   # Tests that spawn specific browsers. These automatically get the browser tag
   # as well.
   chrome: {add_tags: [browser]}
   firefox: {add_tags: [browser]}
   safari: {add_tags: [browser]}
   ie: {add_tags: [browser]}
 ```
 
+This field is not supported in the
+[global configuration file](#global-configuration).
+
 ## Configuring Platforms
 
 There are two different kinds of platform configuration.
 [Operating system configuration](#on_os) cares about the operating system on
 which test runner is running. It sets global configuration for the runner on
 particular OSes. [Test platform configuration](#on_platform), on the other hand,
 cares about the platform the *test* is running on (like the
 [`@OnPlatform` annotation][@OnPlatform]). It sets configuration for particular
 tests that are running on matching platforms.
 
@@ skipping 107 matching lines @@
 presets:
   # Shortcut for running only browser tests.
   browser:
     paths: [test/runner/browser]
 
   # Shortcut for running only Chrome tests.
   chrome:
     filename: "chrome_*_test.dart"
     add_presets: [browser]
 ```
+
+## Global Configuration
+
+The test runner also supports a global configuration file. On Windows, this
+file's local 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.
+
+The global configuration file supports a subset of the fields supported by the
+package-specific configuration file. In general, it doesn't support fields that
+are closely tied to the structure of an individual package. Fields that are not
+supported in the global configuration file say so in their documentation.