Add new test configuration fields.

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 10 matching lines @@
 
   # Sanity tests are quick and verify that nothing is obviously wrong. Declaring
   # the tag allows us to tag tests with it without getting warnings.
   sanity:
 ```
 
 * [Test Configuration](#test-configuration)
   * [`timeout`](#timeout)
   * [`verbose_trace`](#verbose_trace)
   * [`js_trace`](#js_trace)
+  * [`skip`](#skip)
+  * [`test_on`](#test_on)
 * [Runner Configuration](#runner-configuration)
   * [`paths`](#paths)
   * [`filename`](#filename)
   * [`platforms`](#platforms)
   * [`concurrency`](#concurrency)
   * [`pub_serve`](#pub_serve)
   * [`reporter`](#reporter)
 * [Configuring Tags](#configuring-tags)
   * [`add_tags`](#add_tags)
 
@@ skipping 44 matching lines @@
 This boolean field controls whether or not stack traces caused by errors that
 occur while running Dart compiled to JS are converted back to Dart style. This
 conversion uses the source map generated by `dart2js` to approximate the
 original Dart line, column, and in some cases member name for each stack frame.
 It defaults to `false`.
 
 ```yaml
 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
 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.
@@ skipping 141 matching lines @@
   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]}
 ```