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

runtime/vm/dart_api_impl.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 "include/dart_api.h"
 
 #include "vm/bigint_operations.h"
 #include "vm/class_finalizer.h"
 #include "vm/compiler.h"
 #include "vm/dart.h"
@@ skipping 1731 matching lines @@
   const String& str_obj = Api::UnwrapStringHandle(isolate, str);
   if (str_obj.IsExternal()) {
     return str;  // String is already an external string.
   }
   if (str_obj.IsNull()) {
     RETURN_TYPE_ERROR(isolate, str, String);
   }
   if (array == NULL) {
     RETURN_NULL_ERROR(array);
   }
+  if (str_obj.IsCanonical()) {
+    return Api::NewError("Dart_MakeExternalString "
+                         "cannot externalize a read-only string.");

[Anton Muhin, 2012/11/30 21:37:45]

might be worth clarifying what's the difference between immutable and read-only
:)

+  }
   intptr_t str_size = (str_obj.Length() * str_obj.CharSize());
   if ((length < str_size) || (length > String::kMaxElements)) {
     return Api::NewError("Dart_MakeExternalString "
                          "expects argument length to be in the range"
                          "[%"Pd"..%"Pd"].",
                          str_size, String::kMaxElements);
   }
   return Api::NewHandle(isolate,
                         str_obj.MakeExternal(array, length, peer, cback));
 }
@@ skipping 2689 matching lines @@
   }
   {
     NoGCScope no_gc;
     RawObject* raw_obj = obj.raw();
     isolate->heap()->SetPeer(raw_obj, peer);
   }
   return Api::Success(isolate);
 }
 
 }  // namespace dart