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

runtime/bin/process_macos.cc

Index: runtime/bin/process_macos.cc
diff --git a/runtime/bin/process_macos.cc b/runtime/bin/process_macos.cc
index 6c0d451712f5bf8f2a48b82a945c9ff00430ce89..582a8eae1ad14e4f1847ee9dc25038123aae2460 100644
--- a/runtime/bin/process_macos.cc
+++ b/runtime/bin/process_macos.cc
@@ -1076,7 +1076,10 @@ intptr_t Process::SetSignalHandler(intptr_t signal) {
 }
 
 
-void Process::ClearSignalHandler(intptr_t signal) {
+void Process::ClearSignalHandler(intptr_t signal, Dart_Port port) {
+  // Either the port is illegal or there is no current isolate, but not both.
+  ASSERT((port != ILLEGAL_PORT) || (Dart_CurrentIsolate() == NULL));
+  ASSERT((port == ILLEGAL_PORT) || (Dart_CurrentIsolate() != NULL));
   signal = SignalMap(signal);
   if (signal == -1) {
     return;
@@ -1088,7 +1091,7 @@ void Process::ClearSignalHandler(intptr_t signal) {
   while (handler != NULL) {
     bool remove = false;
     if (handler->signal() == signal) {
-      if (handler->port() == Dart_GetMainPortId()) {
+      if ((port == ILLEGAL_PORT) || (handler->port() == port)) {
         if (signal_handlers == handler) {
           signal_handlers = handler->next();
         }