Warn when an unsupported platform is passed.

test/io.dart

Index: test/io.dart
diff --git a/test/io.dart b/test/io.dart
index 1db00ed72b813afc1b20674cbfdecb255dda5914..f7ead650692c26604599182f08e457ca574c7e79 100644
--- a/test/io.dart
+++ b/test/io.dart
@@ -33,6 +33,9 @@ final String noSuchFileMessage = Platform.isWindows
 final _servingRegExp =
     new RegExp(r'^Serving myapp [a-z]+ on http://localhost:(\d+)$');
 
+/// An operating system name that's different than the current operating system.
+final otherOS = Platform.isWindows ? "mac-os" : "windows";
+
 /// A future that will return the port of a pub serve instance run via
 /// [runPubServe].
 ///