Fix several Windows issues.

test/run/forwards_signal_posix_test.dart

Index: test/run/forwards_signal_posix_test.dart
diff --git a/test/run/forwards_signal_posix_test.dart b/test/run/forwards_signal_posix_test.dart
index 1eb6f90fcf05cc9bc791efd2bf5ecfbb77ef4f3b..d09a584b9a82275c557b5df78ff499c6ee2de62c 100644
--- a/test/run/forwards_signal_posix_test.dart
+++ b/test/run/forwards_signal_posix_test.dart
@@ -2,6 +2,9 @@
 // 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.
 
+// Windows doesn't support sending signals.
+@TestOn('!windows')
+
 import 'dart:io';
 
 import '../descriptor.dart' as d;