Allow internal usage of SIGQUIT signal listening.

runtime/bin/process_patch.dart

Index: runtime/bin/process_patch.dart
diff --git a/runtime/bin/process_patch.dart b/runtime/bin/process_patch.dart
index 0f5f8ac9f17867f511fb9fbea567d78a164680c3..970bb659a2caeb592fab6552d14f104c96902140 100644
--- a/runtime/bin/process_patch.dart
+++ b/runtime/bin/process_patch.dart
@@ -139,6 +139,10 @@ patch class _ProcessUtils {
       throw new SignalException(
           "Listening for signal $signal is not supported");
     }
+    return _watchSignalInternal(signal);
+  }
+
+  static Stream<ProcessSignal> _watchSignalInternal(ProcessSignal signal) {
     if (_signalControllers[signal._signalNumber] == null) {
       _signalControllers[signal._signalNumber] = new _SignalController(signal);
     }