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)
@@ -10498,14 +10498,16 @@
                                 void* peer,
                                 Dart_PeerFinalizer cback) const {
   ASSERT(array != NULL);
+  NoGCScope no_gc;

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

Why did this move?  Was it scoped incorrectly before?  I like having the
NoGCScope be as narrow as possible to make it easy to understand the invariants
being preserved.

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

It needs to be before we access any of the stuff from the object and cache in
local variables. We were caching
length, class id etc. and these could potentially change if somehow a GC is
triggered.

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

   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_;
 
+  Heap* vm_isolate_heap = Dart::vm_isolate()->heap();
+  ASSERT(!vm_isolate_heap->Contains(reinterpret_cast<uword>(raw_ptr())));
   if (class_id == kOneByteStringCid) {
     used_size = ExternalOneByteString::InstanceSize();
     original_size = OneByteString::InstanceSize(str_length);