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

runtime/vm/object.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/object.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/bigint_operations.h"
@@ skipping 10479 matching lines @@
   weak_ref->set_raw(referent);
   weak_ref->set_peer(peer);
   weak_ref->set_callback(callback);
 }
 
 
 RawString* String::MakeExternal(void* array,
                                 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);
     ASSERT(original_size >= used_size);
 
     // Copy the data into the external array.
     if (str_length > 0) {
       memmove(array, OneByteString::CharAddr(*this, 0), str_length);
     }
 
@@ skipping 1653 matching lines @@
   }
   return result.raw();
 }
 
 
 const char* WeakProperty::ToCString() const {
   return "_WeakProperty";
 }
 
 }  // namespace dart