Add an option to run skipped tests.

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 22 matching lines @@
 * [Runner Configuration](#runner-configuration)
   * [`paths`](#paths)
   * [`filename`](#filename)
   * [`names`](#names)
   * [`plain_names`](#plain_names)
   * [`include_tags`](#include_tags)
   * [`exclude_tags`](#exclude_tags)
   * [`platforms`](#platforms)
   * [`concurrency`](#concurrency)
   * [`pause_after_load`](#pause_after_load)
+  * [`run_skipped`](#run_skipped)
   * [`pub_serve`](#pub_serve)
   * [`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)
@@ skipping 267 matching lines @@
 
 ```yaml
 presets:
   # Pass "-P debug" to enable debugging configuration
   debug:
     pause_after_load: true
     exclude_tags: undebuggable
     reporter: expanded
 ```
 
+### `run_skipped`
+
+This field indicates that the test runner should run tests even if they're
+marked as skipped.
+
+This is usually used in a [preset](#configuration-presets).
+
+```yaml
+presets:
+  # Pass "-P all" to run all tests
+  debug:
+    run_skipped: true
+    paths: ["test/", "extra_test/"]
+```
+
 ### `pub_serve`
 
 This field indicates that the test runner should run against a `pub serve`
 instance by default, and provides the port number for that instance. Note that
 if there is no `pub serve` instance running at that port, running the tests will
 fail by default.
 
 ```yaml
 pub_serve: 8081
 ```
@@ skipping 226 matching lines @@
 
 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.