Add flag to disable string externalization

runtime/vm/flow_graph_type_propagator.cc

Index: runtime/vm/flow_graph_type_propagator.cc
diff --git a/runtime/vm/flow_graph_type_propagator.cc b/runtime/vm/flow_graph_type_propagator.cc
index 20ccd3a12e1351477b370adf1bda8004ef146b0b..b1d188aef4ef11ead3b9c9e24ebb12c2dbbde78f 100644
--- a/runtime/vm/flow_graph_type_propagator.cc
+++ b/runtime/vm/flow_graph_type_propagator.cc
@@ -270,7 +270,7 @@ void FlowGraphTypePropagator::VisitGuardFieldClass(
   const intptr_t cid = guard->field().guarded_cid();
   if ((cid == kIllegalCid) ||
       (cid == kDynamicCid) ||
-      !CheckClassInstr::IsImmutableClassId(cid)) {
+      Field::IsExternalizableCid(cid)) {
     return;
   }
 
@@ -782,7 +782,7 @@ CompileType ConstantInstr::ComputeType() const {
   }
 
   intptr_t cid = value().GetClassId();
-  if (!CheckClassInstr::IsImmutableClassId(cid)) {
+  if (Field::IsExternalizableCid(cid)) {
     cid = kDynamicCid;
   }
 
@@ -965,7 +965,7 @@ CompileType LoadStaticFieldInstr::ComputeType() const {
       cid = obj.GetClassId();
     }
   }
-  if (!CheckClassInstr::IsImmutableClassId(cid)) {
+  if (Field::IsExternalizableCid(cid)) {
     cid = kDynamicCid;
   }
   return CompileType(is_nullable, cid, abstract_type);