Add debug checks to catch PropertyCell::cast failures.

src/runtime/runtime-object.cc

Index: src/runtime/runtime-object.cc
diff --git a/src/runtime/runtime-object.cc b/src/runtime/runtime-object.cc
index 78b379e012b018afdad616b620cda521171a03aa..0818b7758b1e13ddc2a76400431836867a5db674 100644
--- a/src/runtime/runtime-object.cc
+++ b/src/runtime/runtime-object.cc
@@ -605,6 +605,7 @@ RUNTIME_FUNCTION(Runtime_KeyedGetProperty) {
             (dictionary->DetailsAt(entry).type() == DATA)) {
           Object* value = dictionary->ValueAt(entry);
           if (!receiver->IsGlobalObject()) return value;
+          DCHECK(value->IsPropertyCell());
           value = PropertyCell::cast(value)->value();
           if (!value->IsTheHole()) return value;
           // If value is the hole (meaning, absent) do the general lookup.