Warn when an unsupported platform is passed.

README.md

 `test` provides a standard way of writing and running tests in Dart.
 
 * [Writing Tests](#writing-tests)
 * [Running Tests](#running-tests)
   * [Restricting Tests to Certain Platforms](#restricting-tests-to-certain-platforms)
   * [Platform Selectors](#platform-selectors)
   * [Running Tests on Dartium](#running-tests-on-dartium)
 * [Asynchronous Tests](#asynchronous-tests)
 * [Running Tests With Custom HTML](#running-tests-with-custom-html)
 * [Configuring Tests](#configuring-tests)
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 }
 ```
 
 [TestOn]: http://www.dartdocs.org/documentation/test/latest/index.html#test/test.TestOn
 
 The string you pass to `@TestOn` is what's called a "platform selector", and it
 specifies exactly which platforms a test can run on. It can be as simple as the
 name of a platform, or a more complex Dart-like boolean expression involving
 these platform names.
 
+You can also declare that your entire package only works on certain platforms by
+adding a [`test_on` field][test_on] to your package config file.
+
+[test_on]: https://github.com/dart-lang/test/blob/master/doc/package_config.md#test_on
+
 ### Platform Selectors
 
 Platform selectors use the [boolean selector syntax][] defined in the
 [`boolean_selector` package][boolean_selector], which is a subset of Dart's
 expression syntax that only supports boolean operations. The following
 identifiers are defined:
 
 [boolean selector syntax]: https://github.com/dart-lang/boolean_selector/blob/master/README.md
 
 [boolean_selector]: https://pub.dartlang.org/packages/boolean_selector
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 available to tests.
 
 [api]: http://www.dartdocs.org/documentation/test/latest/index.html
 
 The test runner also supports a machine-readable JSON-based reporter. This
 reporter allows the test runner to be wrapped and its progress presented in
 custom ways (for example, in an IDE). See [the protocol documentation][json] for
 more details.
 
 [json]: https://github.com/dart-lang/test/blob/master/doc/json_reporter.md