Update isolate documentation.

runtime/vm/snapshot.cc

Index: runtime/vm/snapshot.cc
diff --git a/runtime/vm/snapshot.cc b/runtime/vm/snapshot.cc
index c05a8faeef8118e378f54e58d788d6d4944223ef..b7909ac64fadcbbb16ebe84778df5973cbbd5cb7 100644
--- a/runtime/vm/snapshot.cc
+++ b/runtime/vm/snapshot.cc
@@ -1365,13 +1365,13 @@ void SnapshotWriter::ArrayWriteTo(intptr_t object_id,
 void SnapshotWriter::CheckIfSerializable(RawClass* cls) {
   if (Class::IsSignatureClass(cls)) {
     // We do not allow closure objects in an isolate message.
-    SetWriteException(Exceptions::kArgument,
+    SetWriteException(Exceptions::kIsolateMessage,
                       "Illegal argument in isolate message"
                       " : (object is a closure)");
   }
   if (cls->ptr()->num_native_fields_ != 0) {
     // We do not allow objects with native fields in an isolate message.
-    SetWriteException(Exceptions::kArgument,
+    SetWriteException(Exceptions::kIsolateMessage,
                       "Illegal argument in isolate message"
                       " : (object extends NativeWrapper)");
   }