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

runtime/vm/atomic.h

Index: runtime/vm/atomic.h
diff --git a/runtime/vm/atomic.h b/runtime/vm/atomic.h
index 59a45ace74cc42b1d22c7bc6ee4aad92cb9542c5..bd489d0de36aec88030f1bc5edd88715e33db446 100644
--- a/runtime/vm/atomic.h
+++ b/runtime/vm/atomic.h
@@ -20,7 +20,12 @@ class AtomicOperations : public AllStatic {
   // NOTE: Not to be used for any atomic operations involving memory locations
   // that are accessed by generated code.
   static uintptr_t FetchAndIncrement(uintptr_t* p);
-  static uintptr_t FetchAndIncrementBy(intptr_t* p, intptr_t value);
+
+  // Atomically increment the value at p by 'value'.
+  //
+  // NOTE: Not to be used for any atomic operations involving memory locations
+  // that are accessed by generated code.
+  static void IncrementBy(intptr_t* p, intptr_t value);
 
   // Atomically fetch the value at p and decrement the value at p.
   // Returns the original value at p.
@@ -28,7 +33,12 @@ class AtomicOperations : public AllStatic {
   // NOTE: Not to be used for any atomic operations involving memory locations
   // that are accessed by generated code.
   static uintptr_t FetchAndDecrement(uintptr_t* p);
-  static uintptr_t FetchAndDecrementBy(intptr_t* p, intptr_t value);
+
+  // Atomically decrement the value at p by 'value'.
+  //
+  // NOTE: Not to be used for any atomic operations involving memory locations
+  // that are accessed by generated code.
+  static void DecrementBy(intptr_t* p, intptr_t value);
 
   // Atomically compare *ptr to old_value, and if equal, store new_value.
   // Returns the original value at ptr.