Add pause_after_load to the config file.

doc/package_config.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 21 matching lines @@
   * [`test_on`](#test_on)
 * [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)
   * [`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 224 matching lines @@
 This field indicates the default number of test suites to run in parallel. More
 parallelism can improve the overall speed of running tests up to a point, but
 eventually it just adds more memory overhead without any performance gain. This
 defaults to approximately half the number of processors on the current machine.
 If it's set to 1, only one test suite will run at a time.
 
 ```yaml
 concurrency: 3
 ```
 
+### `pause_after_load`
+
+This field indicates that the test runner should pause for debugging after each
+test suite is loaded but before its tests are executed. If it's set,
+[`concurrency`](#concurrency) is automatically set to 1 and
+[`timeout`](#timeout) is automatically set to `none`.
+
+This is usually used in a [preset](#configuration-presets).
+
+```yaml
+presets:
+  # Pass "-P debug" to enable debugging configuration
+  debug:
+    pause_after_load: true
+    exclude_tags: undebuggable
+    reporter: expanded
+```
+
 ### `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 207 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]
 ```