Do not reuse tags_ field to store next_ pointer of FreeListElement.

runtime/vm/class_table.cc

Index: runtime/vm/class_table.cc
diff --git a/runtime/vm/class_table.cc b/runtime/vm/class_table.cc
index 6917b1d81bf5ecb6db1e0e5e9bfe9800b0e033f3..6144a9a31b3a6c3aa98105147a8c1d4a7b1b77e5 100644
--- a/runtime/vm/class_table.cc
+++ b/runtime/vm/class_table.cc
@@ -4,6 +4,7 @@
 
 #include "vm/class_table.h"
 #include "vm/flags.h"
+#include "vm/freelist.h"
 #include "vm/object.h"
 #include "vm/raw_object.h"
 #include "vm/visitor.h"
@@ -27,6 +28,7 @@ ClassTable::ClassTable()
     for (intptr_t i = kObject; i < kInstance; i++) {
       table_[i] = vm_class_table->At(i);
     }
+    table_[kFreeListElement] = vm_class_table->At(kFreeListElement);
     table_[kNullClassId] = vm_class_table->At(kNullClassId);
     table_[kDynamicClassId] = vm_class_table->At(kDynamicClassId);
     table_[kVoidClassId] = vm_class_table->At(kVoidClassId);