Add flag to disable string externalization

runtime/vm/constant_propagator.cc

Index: runtime/vm/constant_propagator.cc
diff --git a/runtime/vm/constant_propagator.cc b/runtime/vm/constant_propagator.cc
index 03ab8ae7c39aaac71cd94b8d91ebd85cd255fbc0..b8e7c2a014b43ec2cbdcafdc657cb151f82b5579 100644
--- a/runtime/vm/constant_propagator.cc
+++ b/runtime/vm/constant_propagator.cc
@@ -323,7 +323,7 @@ void ConstantPropagator::VisitRedefinition(RedefinitionInstr* instr) {
   // class ids. Otherwise LICM might potentially hoist incorrect code.
   const Object& value = instr->value()->definition()->constant_value();
   if (IsConstant(value) &&
-      CheckClassInstr::IsImmutableClassId(value.GetClassId())) {
+      !Field::IsExternalizableCid(value.GetClassId())) {
     SetValue(instr, value);
   } else {
     SetValue(instr, non_constant_);
@@ -805,7 +805,7 @@ void ConstantPropagator::VisitLoadClassId(LoadClassIdInstr* instr) {
   const Object& object = instr->object()->definition()->constant_value();
   if (IsConstant(object)) {
     cid = object.GetClassId();
-    if (CheckClassInstr::IsImmutableClassId(cid)) {
+    if (!Field::IsExternalizableCid(cid)) {
       SetValue(instr, Smi::ZoneHandle(Z, Smi::New(cid)));
       return;
     }