Distinguish internal kill messages from user-initiated kill messages.

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index 7f59321f13e83b0338066adc027559e9602bb455..2d5288ebb17e8db59339398a2ddaf5741585b100 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -125,7 +125,10 @@ class Isolate : public BaseIsolate {
     kAddErrorMsg = 7,
     kDelErrorMsg = 8,
     kErrorFatalMsg = 9,
-    kInterruptMsg = 10,
+
+    // Internal message ids.
+    kInterruptMsg = 10,     // Break in the debugger.
+    kInternalKillMsg = 11,  // Like kill, but does not run exit listeners, etc.
   };
   // The different Isolate API message priorities for ping and kill messages.
   enum LibMsgPriority {