Respect top-level @TestOn declarations.

README.md

 `unittest` 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/unittest/latest/index.html#unittest/unittest@id_test
 [expect]: http://www.dartdocs.org/documentation/unittest/latest/index.html#unittest/unittest@id_expect
 
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 you don't pass any paths, it will run all the test files in your `test/`
 directory, making it easy to test your entire application at once.
 
 By default, tests are run in the Dart VM, but you can run them in the browser as
 well by passing `pub run unittest:unittest -p chrome path/to/test.dart`.
 `unittest` will take care of starting the browser and loading the tests, and all
 the results will be reported on the command line just like for VM tests. In
 fact, you can even run tests on both platforms with a single command: `pub run
 unittest:unittest -p chrome -p vm path/to/test.dart`.
 
+### Restricting Tests to Certain Platforms
+
+Some test files only make sense to run on particular platforms. They may use
+`dart:html` or `dart:io`, they might test Windows' particular filesystem
+behavior, or they might use a feature that's only available in Chrome. The
+[`@TestOn`][TestOn] annotation makes it easy to declare exactly which platforms
+a test file should run on. Just put it at the top of your file, before any
+`library` or `import` declarations:
+
+```dart
+@TestOn("vm")
+
+import "dart:io";
+
+import "package:unittest/unittest.dart";
+
+void main() {
+  // ...
+}
+```
+
+[TestOn]: http://www.dartdocs.org/documentation/unittest/latest/index.html#unittest/unittest.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.
+
+### Platform Selector Syntax
+
+Platform selectors can contain identifiers, parentheses, and operators. When
+loading a test, each identifier is set to `true` or `false` based on the current
+platform, and the test is only loaded if the platform selector returns `true`.
+The operators `||`, `&&`, `!`, and `? :` all work just like they do in Dart. The
+valid identifiers are:
+
+* `vm`: Whether the test is running on the command-line Dart VM.
+
+* `chrome`: Whether the test is running on Google Chrome.
+
+* `dart-vm`: Whether the test is running on the Dart VM in any context. For now
+  this is identical to `vm`, but it will also be true for Dartium in the future.
+  It's identical to `!js`.
+
+* `browser`: Whether the test is running in any browser.
+
+* `js`: Whether the test has been compiled to JS. This is identical to
+  `!dart-vm`.
+
+* `blink`: Whether the test is running in a browser that uses the Blink
+  rendering engine.
+
+* `windows`: Whether the test is running on Windows. If `vm` is false, this will
+  be `false` as well.
+
+* `mac-os`: Whether the test is running on Mac OS. If `vm` is false, this will
+  be `false` as well.
+
+* `linux`: Whether the test is running on Linux. If `vm` is false, this will be
+  `false` as well.
+
+* `android`: Whether the test is running on Android. If `vm` is false, this will
+  be `false` as well, which means that this *won't* be true if the test is
+  running on an Android browser.
+
+* `posix`: Whether the test is running on a POSIX operating system. This is
+  equivalent to `!windows`.
+
+For example, if you wanted to run a test on every browser but Chrome, you would
+write `@TestOn("browser && !chrome")`.
+
 ## Asynchronous Tests
 
 Tests written with `async`/`await` will work automatically. The test runner
 won't consider the test finished until the returned `Future` completes.
 
 ```dart
 import "dart:async";
 
 import "package:unittest/unittest.dart";
 
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     var stream = new Stream.fromIterable([1, 2, 3]);
 
     stream.listen(expectAsync((number) {
       expect(number, inInclusiveRange(1, 3));
     }, count: 3));
   });
 }
 ```
 
 [expectAsync]: http://www.dartdocs.org/documentation/unittest/latest/index.html#unittest/unittest@id_expectAsync