Warn when an unsupported platform is passed.

test/io.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // TODO(nweiz): Remove this tag when we can get [packageDir] working without it
 // (dart-lang/sdk#24022).
 library test.test.io;
 
 import 'dart:async';
 import 'dart:io';
@@ skipping 15 matching lines @@
 
 /// The platform-specific message emitted when a nonexistent file is loaded.
 final String noSuchFileMessage = Platform.isWindows
     ? "The system cannot find the file specified."
     : "No such file or directory";
 
 /// A regular expression that matches the output of "pub serve".
 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].
 ///
 /// This should only be called after [runPubServe].
 Future<int> get pubServePort => _pubServePortCompleter.future;
 Completer<int> _pubServePortCompleter;
 
 /// The path to the sandbox directory.
 ///
 /// This is only set in tests for which [useSandbox] is active.
@@ skipping 126 matching lines @@
     var match;
     while (match == null) {
       var line = await pub.stdout.next();
       match = _servingRegExp.firstMatch(line);
     }
     _pubServePortCompleter.complete(int.parse(match[1]));
   }, "waiting for pub serve to emit its port number");
 
   return pub;
 }