Drop support for old SDK and shelf versions.

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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 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.**
+instance rather than from the filesystem.
 
 Before using the `--pub-serve` option, add the `test/pub_serve` transformer to
 your `pubspec.yaml`. This transformer adds the necessary bootstrapping code that
 allows the test runner to load your tests properly:
 
 ```yaml
 transformers:
 - test/pub_serve:
     $include: test/**_test{.*,}.dart
 ```
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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!
 ```