Quote comma-separated arguments in the README.

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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 The test runner considers any file that ends with `_test.dart` to be a test
 file. If 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 test:test -p chrome path/to/test.dart`.
 `test` 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
-test:test -p chrome,vm path/to/test.dart`.
+test:test -p "chrome,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:
 
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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!
 ```