Fix race in GC sweeper and isolate reload rollback.

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 81ba9a8009611d4f026b6d8673d657b4457171aa..a00f05e75700ae3baa0892b051a398c71ea9d759 100644
--- a/runtime/vm/isolate_reload.cc
+++ b/runtime/vm/isolate_reload.cc
@@ -1324,10 +1324,11 @@ RawClass* IsolateReloadContext::FindOriginalClass(const Class& cls) {
 
 
 RawClass* IsolateReloadContext::GetClassForHeapWalkAt(intptr_t cid) {
-  if (saved_class_table_ != NULL) {
+  RawClass** class_table = AtomicOperations::LoadRelaxed(&saved_class_table_);
+  if (class_table != NULL) {
     ASSERT(cid > 0);
     ASSERT(cid < saved_num_cids_);
-    return saved_class_table_[cid];
+    return class_table[cid];
   } else {
     return isolate_->class_table()->At(cid);
   }