Change signature of atomic increment/decrement functions to return void.

runtime/vm/atomic_macos.h

Index: runtime/vm/atomic_macos.h
diff --git a/runtime/vm/atomic_macos.h b/runtime/vm/atomic_macos.h
index 1cf951e838aba15ca1471c2af8bd1b4d67109fd2..fd5af872c3f58e438424af42e4c78a70ddee7a8e 100644
--- a/runtime/vm/atomic_macos.h
+++ b/runtime/vm/atomic_macos.h
@@ -21,9 +21,8 @@ inline uintptr_t AtomicOperations::FetchAndIncrement(uintptr_t* p) {
 }
 
 
-inline uintptr_t AtomicOperations::FetchAndIncrementBy(intptr_t* p,
-                                                       intptr_t value) {
-  return __sync_fetch_and_add(p, value);
+inline void AtomicOperations::IncrementBy(intptr_t* p, intptr_t value) {
+  __sync_fetch_and_add(p, value);
 }
 
 
@@ -32,9 +31,8 @@ inline uintptr_t AtomicOperations::FetchAndDecrement(uintptr_t* p) {
 }
 
 
-inline uintptr_t AtomicOperations::FetchAndDecrementBy(intptr_t* p,
-                                                       intptr_t value) {
-  return __sync_fetch_and_sub(p, value);
+inline void AtomicOperations::DecrementBy(intptr_t* p, intptr_t value) {
+  __sync_fetch_and_sub(p, value);
 }