Distinguish internal kill messages from user-initiated kill messages.

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 2366142b7a56dacf9e74fbdcb5ace7e723b849d9..20ca510ad09d14c57d9624f9e4cb5827370db035 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -14226,6 +14226,7 @@ RawUnwindError* UnwindError::New(const String& message, Heap::Space space) {
                                       space);
     NoSafepointScope no_safepoint;
     result ^= raw;
+    result.StoreNonPointer(&result.raw_ptr()->is_user_initiated_, false);
   }
   result.set_message(message);
   return result.raw();
@@ -14237,6 +14238,11 @@ void UnwindError::set_message(const String& message) const {
 }
 
 
+void UnwindError::set_is_user_initiated(bool value) const {
+  StoreNonPointer(&raw_ptr()->is_user_initiated_, value);
+}
+
+
 const char* UnwindError::ToErrorCString() const {
   const String& msg_str = String::Handle(message());
   return msg_str.ToCString();