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

runtime/vm/atomic_linux.h

Index: runtime/vm/atomic_linux.h
diff --git a/runtime/vm/atomic_linux.h b/runtime/vm/atomic_linux.h
index 0103b532e90bb14faba03d4d5ed729359a3c4a75..15f03965f4255ec751bd26b9bd3587d96e1da717 100644
--- a/runtime/vm/atomic_linux.h
+++ b/runtime/vm/atomic_linux.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);
 }