Don't overload FreeListElement for become.

runtime/vm/isolate_reload.cc

Index: runtime/vm/isolate_reload.cc
diff --git a/runtime/vm/isolate_reload.cc b/runtime/vm/isolate_reload.cc
index 02915170900afab0a66db91763b2e38268e95b46..693e579d7135ac44854b9876e7497c0b22751a7f 100644
--- a/runtime/vm/isolate_reload.cc
+++ b/runtime/vm/isolate_reload.cc
@@ -408,7 +408,7 @@ void IsolateReloadContext::CheckpointClasses() {
         class_table->HasValidClassAt(i)) {
       // Copy the class into the saved class table and add it to the set.
       local_saved_class_table[i] = class_table->At(i);
-      if (i != kFreeListElement) {
+      if (i != kFreeListElement && i != kForwardingCorpse) {
         cls = class_table->At(i);
         bool already_present = old_classes_set.Insert(cls);
         ASSERT(!already_present);
@@ -972,7 +972,7 @@ class MarkFunctionsForRecompilation : public ObjectVisitor {
 
   virtual void VisitObject(RawObject* obj) {
     // Free-list elements cannot even be wrapped in handles.
-    if (obj->IsFreeListElement()) {
+    if (obj->IsFreeListElement() || obj->IsForwardingCorpse()) {

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

This seems like a common enough pattern that we might want a utility method.

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

Done.

       return;
     }
     handle_ = obj;