When externalizing a string ensure that the basic tag bits are preserved.

runtime/vm/object.cc

Index: runtime/vm/object.cc
===================================================================
--- runtime/vm/object.cc	(revision 15532)
+++ runtime/vm/object.cc	(working copy)
@@ -10497,15 +10497,16 @@
                                 intptr_t length,
                                 void* peer,
                                 Dart_PeerFinalizer cback) const {
+  NoGCScope no_gc;
   ASSERT(array != NULL);
   intptr_t str_length = this->Length();
   ASSERT(length >= (str_length * this->CharSize()));
   intptr_t class_id = raw()->GetClassId();
   intptr_t used_size = 0;
   intptr_t original_size = 0;
-  uword tags = 0;
-  NoGCScope no_gc;
+  uword tags = raw_ptr()->tags_;
 
+  ASSERT(!IsCanonical());
   if (class_id == kOneByteStringCid) {
     used_size = ExternalOneByteString::InstanceSize();
     original_size = OneByteString::InstanceSize(str_length);