Fix hanging process (kill of parent process leaves child as a zombie)

sdk/lib/_internal/pub/test/run/forwards_signal_posix_test.dart

Index: sdk/lib/_internal/pub/test/run/forwards_signal_posix_test.dart
diff --git a/sdk/lib/_internal/pub/test/run/forwards_signal_posix_test.dart b/sdk/lib/_internal/pub/test/run/forwards_signal_posix_test.dart
index ccfbe580e7a298164fda46dc692f6c184e8d5dab..6b9e2222d1a8d519cef006dbe844fa733db06546 100644
--- a/sdk/lib/_internal/pub/test/run/forwards_signal_posix_test.dart
+++ b/sdk/lib/_internal/pub/test/run/forwards_signal_posix_test.dart
@@ -19,11 +19,11 @@ const SCRIPT = """
 import 'dart:io';
 
 main() {
-  ProcessSignal.SIGHUP.watch().listen(print);
-  ProcessSignal.SIGTERM.watch().listen(print);
-  ProcessSignal.SIGUSR1.watch().listen(print);
-  ProcessSignal.SIGUSR2.watch().listen(print);
-  ProcessSignal.SIGWINCH.watch().listen(print);
+  ProcessSignal.SIGHUP.watch().first.then(print);
+  ProcessSignal.SIGTERM.watch().first.then(print);
+  ProcessSignal.SIGUSR1.watch().first.then(print);
+  ProcessSignal.SIGUSR2.watch().first.then(print);
+  ProcessSignal.SIGWINCH.watch().first.then(print);
 
   print("ready");
 }