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

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
===================================================================
--- runtime/vm/dart_api_impl.cc	(revision 15532)
+++ runtime/vm/dart_api_impl.cc	(working copy)
@@ -1749,6 +1749,11 @@
   if (array == NULL) {
     RETURN_NULL_ERROR(array);
   }
+  Heap* vm_isolate_heap = Dart::vm_isolate()->heap();
+  if (vm_isolate_heap->Contains(reinterpret_cast<uword>(str_obj.raw()))) {

[cshapiro, 2012/11/29 21:03:12]

Is this the most efficient check?  Can we use the canonical bit instead?

[siva, 2012/11/29 22:37:39]

My initial thought was strings in the isolate can also be canonical and checking
the canonical bit would essentially mean that all canonical strings cannot be
externalized.

As discussed off-line I will start conservatively by checking the canonical bit
and if it turns out to be
too restrictive we can re-evaluate.

On 2012/11/29 21:03:12, cshapiro wrote:

+    return Api::NewError("Dart_MakeExternalString "
+                         "cannot externalize a read only string.");

[cshapiro, 2012/11/29 21:03:12]

s/read only/read-only/

[siva, 2012/11/29 22:37:39]

Done.

+  }
   intptr_t str_size = (str_obj.Length() * str_obj.CharSize());
   if ((length < str_size) || (length > String::kMaxElements)) {
     return Api::NewError("Dart_MakeExternalString "