Add IE 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
 
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 * `chrome`: Whether the test is running on Google Chrome.
 
 * `phantomjs`: Whether the test is running on
   [PhantomJS](http://phantomjs.org/).
 
 * `firefox`: Whether the test is running on Mozilla Firefox.
 
 * `safari`: Whether the test is running on Apple Safari.
 
+* `ie`: Whether the test is running on Microsoft Internet Explorer.
+
 * `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`.
 
 * `blink`: Whether the test is running in a browser that uses the Blink
   rendering engine.
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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!
 ```