Use atomic operations to increment/decrement memory usage counts as multiple threads could be alloc…

runtime/vm/atomic_test.cc

Index: runtime/vm/atomic_test.cc
diff --git a/runtime/vm/atomic_test.cc b/runtime/vm/atomic_test.cc
index 2e327a52235817c8b444046a8b9a1625b3a88b80..db9be229f76252105474269783cd0aa7ca824b12 100644
--- a/runtime/vm/atomic_test.cc
+++ b/runtime/vm/atomic_test.cc
@@ -26,6 +26,22 @@ UNIT_TEST_CASE(FetchAndDecrement) {
 }
 
 
+UNIT_TEST_CASE(FetchAndIncrementBy) {
+  intptr_t v = 42;
+  EXPECT_EQ(static_cast<uintptr_t>(42),
+            AtomicOperations::FetchAndIncrementBy(&v, 100));
+  EXPECT_EQ(static_cast<intptr_t>(142), v);
+}
+
+
+UNIT_TEST_CASE(FetchAndDecrementBy) {
+  intptr_t v = 42;
+  EXPECT_EQ(static_cast<uintptr_t>(42),
+            AtomicOperations::FetchAndDecrementBy(&v, 41));
+  EXPECT_EQ(static_cast<intptr_t>(1), v);
+}
+
+
 UNIT_TEST_CASE(LoadRelaxed) {
   uword v = 42;
   EXPECT_EQ(static_cast<uword>(42), AtomicOperations::LoadRelaxed(&v));