Support an @OnPlatform annotation.

README.md

 `test` provides a standard way of writing and running tests in Dart.
 
 ## Writing Tests
 
 Tests are specified using the top-level [`test()`][test] function, and test
 assertions are made using [`expect()`][expect]:
 
 [test]: http://www.dartdocs.org/documentation/test/latest/index.html#test/test@id_test
 [expect]: http://www.dartdocs.org/documentation/test/latest/index.html#test/test@id_expect
 
@@ skipping 385 matching lines @@
       // ...
     }, timeout: new Timeout.factor(2))
   }, timeout: new Timeout(new Duration(minutes: 1)));
 }
 ```
 
 Nested timeouts apply in order from outermost to innermost. That means that
 "even slower test" will take two minutes to time out, since it multiplies the
 group's timeout by 2.
 
+### Platform-Specific Configuration
+
+Sometimes a test may need to be configured differently for different platforms.
+Windows might run your code slower than other platforms, or your DOM
+manipulation might not work right on Safari yet. For these cases, you can use
+the `@OnPlatform` annotation and the `onPlatform` named parameter to `test()`
+and `group()`. For example:
+
+```dart
+@OnPlatform(const {
+  // Give Windows some extra wiggle-room before timing out.
+  "windows": const Timeout.factor(2)
+})
+
+import "package:test/test.dart";
+
+void main() {
+  test("do a thing", () {
+    // ...
+  }, onPlatform: {
+    "safari": new Skip("Safari is currently broken (see #1234)")
+  });
+}
+```
+
+Both the annotation and the parameter take a map. The map's keys are [platform
+selectors](#platform-selector-syntax) which describe the platforms for which the
+specialized configuration applies. Its values are instances of some of the same
+annotation classes that can be used for a suite: `Skip` and `Timeout`. A value
+can also be a list of these values.
+
+If multiple platforms match, the configuration is applied in order from first to
+last, just as they would in nested groups. This means that for configuration
+like duration-based timeouts, the last matching value wins.
+
 ## Testing With `barback`
 
 Packages using the `barback` transformer system may need to test code that's
 created or modified using transformers. The test runner handles this using the
 `--pub-serve` option, which tells it to load the test code from a `pub serve`
 instance rather than from the filesystem. **This feature is only supported on
 Dart `1.9.2` and higher.**
 
 Before using the `--pub-serve` option, add the `test/pub_serve` transformer to
 your `pubspec.yaml`. This transformer adds the necessary bootstrapping code that
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 In this case, the port is `8081`. In another terminal, pass this port to
 `--pub-serve` and otherwise invoke `pub run test:test` as normal:
 
 ```shell
 $ pub run test:test --pub-serve=8081 -p chrome
 "pub serve" is compiling test/my_app_test.dart...
 "pub serve" is compiling test/utils_test.dart...
 00:00 +42: All tests passed!
 ```