Warn when an unsupported platform is passed.

test/runner/test_on_test.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.
 
 @TestOn("vm")
 
 import 'dart:io';
 
 import 'package:scheduled_test/descriptor.dart' as d;
 import 'package:scheduled_test/scheduled_stream.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 import 'package:test/src/util/io.dart';
 
 import '../io.dart';
 
-final _otherOS = Platform.isWindows ? "mac-os" : "windows";
-
 void main() {
   useSandbox();
 
   group("for suite", () {
     test("runs a test suite on a matching platform", () {
       _writeTestFile("vm_test.dart", suiteTestOn: "vm");
 
       var test = runTest(["vm_test.dart"]);
       test.stdout.expect(consumeThrough(contains("All tests passed!")));
       test.shouldExit(0);
@@ skipping 172 matching lines @@
   if (groupTestOn != null) {
     buffer.writeln("  }, testOn: '$groupTestOn');");
   } else {
     buffer.writeln("  });");
   }
 
   buffer.writeln("}");
 
   d.file(filename, buffer.toString()).create();
 }