Don't overload FreeListElement for become.

runtime/vm/service.cc

Index: runtime/vm/service.cc
diff --git a/runtime/vm/service.cc b/runtime/vm/service.cc
index fc78a4879bb26214aa9c91ff5025ea6899a97b91..81f3ffc67c7074a623344f179504f92cb8588f4d 100644
--- a/runtime/vm/service.cc
+++ b/runtime/vm/service.cc
@@ -2217,7 +2217,7 @@ class GetInstancesVisitor : public ObjectGraph::Visitor {
 
   virtual Direction VisitObject(ObjectGraph::StackIterator* it) {
     RawObject* raw_obj = it->Get();
-    if (raw_obj->IsFreeListElement()) {
+    if (raw_obj->IsFreeListElement() || raw_obj->IsForwardingCorpse()) {
       return kProceed;
     }
     Thread* thread = Thread::Current();
@@ -3342,7 +3342,7 @@ class ContainsAddressVisitor : public FindObjectVisitor {
 
   virtual bool FindObject(RawObject* obj) const {
     // Free list elements are not real objects, so skip them.
-    if (obj->IsFreeListElement()) {
+    if (obj->IsFreeListElement() || obj->IsForwardingCorpse()) {

[Cutch, 2016/06/02 14:09:53]

I really think a utility method might be helpful: 

bool IsFakeObject() const;

?

[rmacnak, 2016/06/02 17:57:06]

Done.

       return false;
     }
     uword obj_begin = RawObject::ToAddr(obj);