Ignore deleted handles when processing weak roots.

runtime/vm/gc_marker.cc

Index: runtime/vm/gc_marker.cc
diff --git a/runtime/vm/gc_marker.cc b/runtime/vm/gc_marker.cc
index 2c8c9f2fb73a2b3ca036b0790808b1109efa94f0..a8985cfcf9b94134eff59a82a5cb1faba3bf4a25 100644
--- a/runtime/vm/gc_marker.cc
+++ b/runtime/vm/gc_marker.cc
@@ -197,7 +197,10 @@ class MarkingWeakVisitor : public HandleVisitor {
     WeakPersistentHandle* handle =
         reinterpret_cast<WeakPersistentHandle*>(addr);
     RawObject* raw_obj = handle->raw();
-    ASSERT(raw_obj->IsHeapObject());
+    if (!raw_obj->IsHeapObject()) {
+      // Deleted handles are identified by a Smi as the raw object value.

[siva, 2012/02/15 01:45:07]

We could probably do a Clear() in FreeHandle() that way we would have a
Object::null() in raw_ and the comment would not be needed.

The check here is still valid as you rightly pointed out about the case when the
user sticks a Smi value here.

+      return;
+    }
     if (!raw_obj->IsMarked() && raw_obj->IsOldObject()) {
       WeakPersistentHandle::Finalize(handle);
     }