Warn when an unsupported platform is passed.

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 93 matching lines @@
 [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.
+This field declares which platforms a test supports. It takes a
+[platform selector][] 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 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"}
 ```
 
+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
+```
+
 ## 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]}
 ```