Fix crash when posting a message to a send port with port id 0.

runtime/vm/port.cc

Index: runtime/vm/port.cc
diff --git a/runtime/vm/port.cc b/runtime/vm/port.cc
index cfb9ac87d5d1bfa700d943ffc46b5012fb0a202b..5a7506852cfe941ece58c05bc03907f0e7813704 100644
--- a/runtime/vm/port.cc
+++ b/runtime/vm/port.cc
@@ -25,6 +25,14 @@ Random* PortMap::prng_ = NULL;
 
 
 intptr_t PortMap::FindPort(Dart_Port port) {
+  // ILLEGAL_PORT (0) is used as a sentinel value in Entry.port. The loop below
+  // could return the index to a deleted port when we are searching for
+  // port id ILLEGAL_PORT. Return -1 immediately to indicate the port
+  // does not exist.
+  if (port == ILLEGAL_PORT) {
+    return -1;
+  }
+  ASSERT(port != ILLEGAL_PORT);
   intptr_t index = port % capacity_;
   intptr_t start_index = index;
   Entry entry = map_[index];