Add content shell support.

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 118 matching lines @@
 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.
 
 * `dartium`: Whether the test is running on Dartium.
 
+* `content-shell`: Whether the test is running on the headless Dartium content
+  shell.
+
 * `chrome`: Whether the test is running on Google Chrome.
 
 * `dart-vm`: Whether the test is running on the Dart VM in any context,
   including Dartium. 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`.
 
@@ skipping 16 matching lines @@
 * `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")`.
 
 ### Running Tests on Dartium
 
 Tests can be run on [Dartium][] by passing the `-p dartium` flag. If you're
 using the Dart Editor, the test runner will be able to find Dartium
-automatically. However, since it usually isn't installed on a system-wide basis,
-the test runner may not otherwise be able to find the Dartium executable. To use
-it without the Editor, make sure there's an executable called `dartium` (on Mac
-OS or Linux) or `dartium.exe` (on Windows) on your system path.
+automatically. On Mac OS, you can also [install it using Homebrew][homebrew].
+Otherwise, make sure there's an executable called `dartium` (on Mac OS or Linux)
+or `dartium.exe` (on Windows) on your system path.
 
 [Dartium]: https://www.dartlang.org/tools/dartium/
+[homebrew]: https://github.com/dart-lang/homebrew-dart
+
+Similarly, tests can be run on the headless Dartium content shell by passing `-p
+content-shell`. The content shell is installed along with Dartium when using
+Homebrew. Otherwise, you can downloaded it manually [from this
+page][content_shell]; if you do, make sure the executable named `content_shell`
+(on Mac OS or Linux) or `content_shell.exe` (on Windows) is on your system path.
+
+[content_shell]: http://gsdview.appspot.com/dart-archive/channels/stable/release/latest/dartium/
 
 [In the future][issue 63], there will be a more explicit way to configure the
-location of the executable.
+location of both the Dartium and content shell executables.
 
 [issue 63]: https://github.com/dart-lang/test/issues/63
 
 ## 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";
@@ skipping 102 matching lines @@
 
 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!
 ```