[vm] Remove Dart_MakeExternalString and --support-externalizable-strings

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 7fe14fa89105a9f36baee0e0ee85317943376d2a..046cc5d05c0c346ebf34fee11d616f465e5fe28b 100644
--- a/runtime/vm/constant_propagator.cc
+++ b/runtime/vm/constant_propagator.cc
@@ -289,11 +289,8 @@ void ConstantPropagator::VisitPhi(PhiInstr* instr) {
 }
 
 void ConstantPropagator::VisitRedefinition(RedefinitionInstr* instr) {
-  // Ensure that we never remove redefinition of a constant unless we are also
-  // are guaranteed to fold away code paths that correspond to non-matching
-  // class ids. Otherwise LICM might potentially hoist incorrect code.
   const Object& value = instr->value()->definition()->constant_value();
-  if (IsConstant(value) && !Field::IsExternalizableCid(value.GetClassId())) {
+  if (IsConstant(value)) {
     SetValue(instr, value);
   } else {
     SetValue(instr, non_constant_);
@@ -736,10 +733,8 @@ void ConstantPropagator::VisitLoadClassId(LoadClassIdInstr* instr) {
   const Object& object = instr->object()->definition()->constant_value();
   if (IsConstant(object)) {
     cid = object.GetClassId();
-    if (!Field::IsExternalizableCid(cid)) {
-      SetValue(instr, Smi::ZoneHandle(Z, Smi::New(cid)));
-      return;
-    }
+    SetValue(instr, Smi::ZoneHandle(Z, Smi::New(cid)));
+    return;
   }
   SetValue(instr, non_constant_);
 }