Compatible atomics: volatile for LoadRelaxed, replace FetchAndAdd with FetchAndDecrement.

runtime/vm/atomic_test.cc

 // Copyright (c) 2015, 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 "platform/assert.h"
 #include "platform/utils.h"
 #include "vm/atomic.h"
 #include "vm/globals.h"
 #include "vm/unit_test.h"
 
 namespace dart {
 
 UNIT_TEST_CASE(FetchAndIncrement) {
   uintptr_t v = 42;
   EXPECT_EQ(static_cast<uintptr_t>(42),
             AtomicOperations::FetchAndIncrement(&v));
   EXPECT_EQ(static_cast<uintptr_t>(43), v);
 }
 
 
-UNIT_TEST_CASE(FetchAndAdd) {
-  intptr_t v = 42;
-  EXPECT_EQ(42, AtomicOperations::FetchAndAdd(&v, 0));
-  EXPECT_EQ(42, v);
-  EXPECT_EQ(42, AtomicOperations::FetchAndAdd(&v, 2));
-  EXPECT_EQ(44, v);
-  EXPECT_EQ(44, AtomicOperations::FetchAndAdd(&v, -2));
-  EXPECT_EQ(42, v);
+UNIT_TEST_CASE(FetchAndDecrement) {
+  uintptr_t v = 42;
+  EXPECT_EQ(static_cast<uintptr_t>(42),
+            AtomicOperations::FetchAndDecrement(&v));
+  EXPECT_EQ(static_cast<uintptr_t>(41), v);
 }
 
 
 UNIT_TEST_CASE(LoadRelaxed) {
   uword v = 42;
   EXPECT_EQ(static_cast<uword>(42), AtomicOperations::LoadRelaxed(&v));
 }
 
 }  // namespace dart