Improve error handling in the process library.

runtime/bin/eventhandler_macos.cc

Index: runtime/bin/eventhandler_macos.cc
diff --git a/runtime/bin/eventhandler_macos.cc b/runtime/bin/eventhandler_macos.cc
index 7d8d04bbb0385ba1c5d47fb20fbabf9223ac9848..ebfb50902be60efdab9938929b29e110f0950ec8 100644
--- a/runtime/bin/eventhandler_macos.cc
+++ b/runtime/bin/eventhandler_macos.cc
@@ -60,7 +60,7 @@ EventHandlerImplementation::~EventHandlerImplementation() {
 void EventHandlerImplementation::SetPort(intptr_t fd,
                                  Dart_Port dart_port,
                                  intptr_t mask) {
-  assert(fd >= 0);
+  ASSERT(fd >= 0);
   if (fd >= port_map_size_) {
     intptr_t new_port_map_size = port_map_size_;
     do {
@@ -254,7 +254,7 @@ void EventHandlerImplementation::HandleEvents(struct pollfd* pollfds,
       if (event_mask != 0) {
         intptr_t fd = pollfds[i].fd;
         Dart_Port port = PortFor(fd);
-        assert(port != 0);
+        ASSERT(port != 0);
         UnregisterFd(fd);
         Dart_PostIntArray(port, 1, &event_mask);
       }