Reload: Don't free the saved class table while another thread may be reading it.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index 028a130008587d395e2d73e7c75a494339e72ba1..362d276b68e970cb23cd1885b33e7ca497f1e54d 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -1109,6 +1109,10 @@ bool Isolate::ReloadSources(JSONStream* js,
 
 
 void Isolate::DeleteReloadContext() {
+  // Another thread may be in the middle of GetClassForHeapWalkAt.
+  Thread* thread = Thread::Current();
+  SafepointOperationScope safepoint_scope(thread);
+
   delete reload_context_;
   reload_context_ = NULL;
 }