Fix race in GC sweeper and isolate reload rollback.

runtime/vm/isolate_reload.cc

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/isolate_reload.h"
 
 #include "vm/become.h"
 #include "vm/bit_vector.h"
 #include "vm/code_generator.h"
 #include "vm/compiler.h"
@@ skipping 1306 matching lines @@
   return !FLAG_reload_force_rollback && !HasReasonsForCancelling();
 }
 
 
 RawClass* IsolateReloadContext::FindOriginalClass(const Class& cls) {
   return MappedClass(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);
   }
 }
 
 
 RawLibrary* IsolateReloadContext::saved_root_library() const {
   return saved_root_library_;
 }
 
@@ skipping 324 matching lines @@
         ASSERT(!super_cls.IsNull());
         super_cls.AddDirectSubclass(cls);
       }
     }
   }
 }
 
 #endif  // !PRODUCT
 
 }  // namespace dart