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

runtime/bin/process_linux.cc

Index: runtime/bin/process_linux.cc
diff --git a/runtime/bin/process_linux.cc b/runtime/bin/process_linux.cc
index 4e1ae8c73d749e5fdaff01fe7e2c35ce36dd7b6b..342d600e498a3ccebe6b8f3f21e3ead4e3c15527 100644
--- a/runtime/bin/process_linux.cc
+++ b/runtime/bin/process_linux.cc
@@ -999,7 +999,10 @@ 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));
   ThreadSignalBlocker blocker(kSignalsCount, kSignals);
   MutexLocker lock(signal_mutex);
   SignalInfo* handler = signal_handlers;
@@ -1007,7 +1010,7 @@ 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();
         }