[fuchsia] Fix shutdown crash

runtime/bin/process_fuchsia.cc

 // Copyright (c) 2016, 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.
 
 #if !defined(DART_IO_DISABLED)
 
 #include "platform/globals.h"
 #if defined(HOST_OS_FUCHSIA)
 
 #include "bin/process.h"
@@ skipping 790 matching lines @@
     return -1;
   }
   ProcessStarter starter(path, arguments, arguments_length, working_directory,
                          environment, environment_length, mode, in, out, err,
                          id, exit_event, os_error_message);
   return starter.Start();
 }
 
 
 intptr_t Process::SetSignalHandler(intptr_t signal) {
-  UNIMPLEMENTED();
+  errno = ENOSYS;
   return -1;
 }
 
 
 void Process::ClearSignalHandler(intptr_t signal) {
-  UNIMPLEMENTED();
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(HOST_OS_FUCHSIA)
 
 #endif  // !defined(DART_IO_DISABLED)