Ignore deleted handles when processing weak roots.

runtime/vm/scavenger.cc

Index: runtime/vm/scavenger.cc
diff --git a/runtime/vm/scavenger.cc b/runtime/vm/scavenger.cc
index d322981bafa12f48dc266446600044ae33e793f8..e87139ec92dcf059bc2db06f3ab6c90acb92f332 100644
--- a/runtime/vm/scavenger.cc
+++ b/runtime/vm/scavenger.cc
@@ -140,6 +140,10 @@ class ScavengerWeakVisitor : public HandleVisitor {
     WeakPersistentHandle* handle =
         reinterpret_cast<WeakPersistentHandle*>(addr);
     RawObject* raw_obj = handle->raw();
+    if (!raw_obj->IsHeapObject()) {
+      // Deleted handles are identified by a Smi as the raw object value.
+      return;
+    }
     uword raw_addr = RawObject::ToAddr(raw_obj);
     if (scavenger_->from_->Contains(raw_addr)) {
       uword header = *reinterpret_cast<uword*>(raw_addr);