AOT: Add a separate switchable call state for unlinked calls.

runtime/vm/raw_object_snapshot.cc

Index: runtime/vm/raw_object_snapshot.cc
diff --git a/runtime/vm/raw_object_snapshot.cc b/runtime/vm/raw_object_snapshot.cc
index 7829e95586f585341d5ada35d4d42a430c744a11..f345a19deba1972574eaaeb0291e4163b803c7d3 100644
--- a/runtime/vm/raw_object_snapshot.cc
+++ b/runtime/vm/raw_object_snapshot.cc
@@ -1543,6 +1543,24 @@ void RawSingleTargetCache::WriteTo(SnapshotWriter* writer,
 }
 
 
+RawUnlinkedCall* UnlinkedCall::ReadFrom(SnapshotReader* reader,
+                                        intptr_t object_id,
+                                        intptr_t tags,
+                                        Snapshot::Kind kind,
+                                        bool as_reference) {
+  UNREACHABLE();
+  return UnlinkedCall::null();
+}
+
+
+void RawUnlinkedCall::WriteTo(SnapshotWriter* writer,
+                              intptr_t object_id,
+                              Snapshot::Kind kind,
+                              bool as_reference) {
+  UNREACHABLE();
+}
+
+
 RawICData* ICData::ReadFrom(SnapshotReader* reader,
                             intptr_t object_id,
                             intptr_t tags,