Fix the following tsan errors:

runtime/vm/class_table.cc

 // Copyright (c) 2012, 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/class_table.h"
 
 #include "vm/atomic.h"
 #include "vm/flags.h"
 #include "vm/freelist.h"
 #include "vm/growable_array.h"
@@ skipping 106 matching lines @@
   intptr_t index = cls.id();
   if (index != kIllegalCid) {
     ASSERT(index > 0);
     ASSERT(index < kNumPredefinedCids);
     ASSERT(table_[index] == 0);
     ASSERT(index < capacity_);
     table_[index] = cls.raw();
     // Add the vtable for this predefined class into the static vtable registry
     // if it has not been setup yet.
     cpp_vtable cls_vtable = cls.handle_vtable();
-    AtomicOperations::CompareAndSwapWord(&(Object::builtin_vtables_[index]), 0,
-                                         cls_vtable);
-    ASSERT(Object::builtin_vtables_[index] == cls_vtable);
+    cpp_vtable old_cls_vtable = AtomicOperations::CompareAndSwapWord(
+        &(Object::builtin_vtables_[index]), 0, cls_vtable);
+    if (old_cls_vtable != 0) {
+      ASSERT(old_cls_vtable == cls_vtable);
+    }
   } else {
     if (top_ == capacity_) {
       // Grow the capacity of the class table.
       // TODO(koda): Add ClassTable::Grow to share code.
       intptr_t new_capacity = capacity_ + capacity_increment_;
       RawClass** new_table = reinterpret_cast<RawClass**>(
           malloc(new_capacity * sizeof(RawClass*)));  // NOLINT
       memmove(new_table, table_, capacity_ * sizeof(RawClass*));
 #ifndef PRODUCT
       ClassHeapStats* new_stats_table = reinterpret_cast<ClassHeapStats*>(
@@ skipping 449 matching lines @@
 void ClassTable::UpdateLiveNew(intptr_t cid, intptr_t size) {
   ClassHeapStats* stats = PreliminaryStatsAt(cid);
   ASSERT(stats != NULL);
   ASSERT(size >= 0);
   stats->post_gc.AddNew(size);
 }
 #endif  // !PRODUCT
 
 
 }  // namespace dart