Add Windows support.

test/runner/signal_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")
+// Windows doesn't support sending signals.
+@TestOn("vm && !windows")
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
 import 'package:test/test.dart';
 import 'package:test/src/util/io.dart';
 
 import '../io.dart';
@@ skipping 252 matching lines @@
       });
     });
   });
 }
 
 Future<Process> _startTest(List<String> args) {
   new Directory(_tempDir).create();
   return startTest(args, workingDirectory: _sandbox,
       environment: {"_UNITTEST_TEMP_DIR": _tempDir});
 }