[dart:io] Fixes a crash in VM shutdown when signals are watched

tests/standalone/io/signals_exception_test.dart

Index: tests/standalone/io/signals_exception_test.dart
diff --git a/tests/standalone/io/signals_exception_test.dart b/tests/standalone/io/signals_exception_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..5c3b926ef9ea6fb84b360a860910273bfba5337a
--- /dev/null
+++ b/tests/standalone/io/signals_exception_test.dart
@@ -0,0 +1,15 @@
+// Copyright (c) 2017, 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.
+
+import "dart:io";
+
+import "package:expect/expect.dart";
+
+main() {
+  var ps = ProcessSignal.SIGINT.watch().listen((signal) {
+    Expect.fail("Unreachable");
+  });
+  throw "Death"; //# 01: runtime error
+  ps.cancel();
+}