Resolution for issue #5092: Unit test handle checks consider dangling handles to be valid.

runtime/vm/thread_registry.cc

Index: runtime/vm/thread_registry.cc
diff --git a/runtime/vm/thread_registry.cc b/runtime/vm/thread_registry.cc
index c2c81a4253b9388d2ef1082fe9f2fbaa03f67ce2..828d53f7b945f4fc056ad8f088f579d61537cc2e 100644
--- a/runtime/vm/thread_registry.cc
+++ b/runtime/vm/thread_registry.cc
@@ -105,6 +105,19 @@ void ThreadRegistry::PrintJSON(JSONStream* stream) const {
 #endif
 
 
+bool ThreadRegistry::IsValidHandle(Dart_Handle handle) const {
+  MonitorLocker ml(threads_lock());
+  Thread* current = active_list_;
+  while (current != NULL) {
+    if (current->IsValidHandle(handle)) {
+      return true;
+    }
+    current = current->next_;
+  }
+  return false;
+}
+
+
 intptr_t ThreadRegistry::CountZoneHandles() const {
   MonitorLocker ml(threads_lock());
   intptr_t count = 0;