Add new test configuration fields.

doc/package_config.md

Index: doc/package_config.md
diff --git a/doc/package_config.md b/doc/package_config.md
index 82aaf68a11e11e4e69a73389069fa974e5ad3313..23db939b8101d7c846d29b8c8ae26c94e98e1189 100644
--- a/doc/package_config.md
+++ b/doc/package_config.md
@@ -28,6 +28,8 @@ tags:
   * [`timeout`](#timeout)
   * [`verbose_trace`](#verbose_trace)
   * [`js_trace`](#js_trace)
+  * [`skip`](#skip)
+  * [`test_on`](#test_on)
 * [Runner Configuration](#runner-configuration)
   * [`paths`](#paths)
   * [`filename`](#filename)
@@ -92,6 +94,36 @@ It defaults to `false`.
 js_trace: true
 ```
 
+### `skip`
+
+This field controls whether or not tests are skipped. It's usually applied to
+[specific tags](#configuring-tags) rather than used at the top level. Like the
+`skip` parameter for [`test()`][test], it can either be a boolean indicating
+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."
+```
+
+### `test_on`
+
+This field declares which platforms a test supports. It's usually applied to
+[specific tags](#configuring-tags) rather than used at the top level. It takes a
+[platform selector][] and only allows a test to run on platforms that match the
+selector.
+
+[platform selector]: 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"}
+```
+
 ## Runner Configuration
 
 Unlike [test configuration](#test-configuration), runner configuration affects