Type and constant propagator should guard against mutable class ids.

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 890f5408f24a18725fcd650a898ebe6652e42f48..004c4e68c253eedab0c80556d8a48e3809cbce10 100644
--- a/runtime/vm/constant_propagator.cc
+++ b/runtime/vm/constant_propagator.cc
@@ -783,10 +783,14 @@ void ConstantPropagator::VisitLoadClassId(LoadClassIdInstr* instr) {
     SetValue(instr, Smi::ZoneHandle(Z, Smi::New(cid)));
     return;
   }
+
   const Object& object = instr->object()->definition()->constant_value();
   if (IsConstant(object)) {
-    SetValue(instr, Smi::ZoneHandle(Z, Smi::New(object.GetClassId())));
-    return;
+    cid = object.GetClassId();
+    if (CheckClassInstr::IsImmutableClassId(cid)) {
+      SetValue(instr, Smi::ZoneHandle(Z, Smi::New(cid)));
+      return;
+    }
   }
   SetValue(instr, non_constant_);
 }