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

runtime/vm/atomic_win.h

Index: runtime/vm/atomic_win.h
diff --git a/runtime/vm/atomic_win.h b/runtime/vm/atomic_win.h
index 0c31e592aa6b4d1bee4cb70f00f0df0c2562b8fb..4ef539c3a3f3f639496b4fe2947653b505c95ca8 100644
--- a/runtime/vm/atomic_win.h
+++ b/runtime/vm/atomic_win.h
@@ -28,13 +28,13 @@ inline uintptr_t AtomicOperations::FetchAndIncrement(uintptr_t* p) {
 }
 
 
-inline intptr_t AtomicOperations::FetchAndAdd(intptr_t* p, intptr_t delta) {
+inline uintptr_t AtomicOperations::FetchAndDecrement(uintptr_t* p) {
 #if defined(HOST_ARCH_X64)
-  return static_cast<intptr_t>(
-      InterlockedAdd64(reinterpret_cast<LONGLONG*>(p), delta)) - delta;
+  return static_cast<uintptr_t>(
+      InterlockedDecrement64(reinterpret_cast<LONGLONG*>(p))) + 1;
 #elif defined(HOST_ARCH_IA32)
-  return static_cast<intptr_t>(
-      InterlockedAdd(reinterpret_cast<LONG*>(p), delta)) - delta;
+  return static_cast<uintptr_t>(
+      InterlockedDecrement(reinterpret_cast<LONG*>(p))) + 1;
 #else
 #error Unsupported host architecture.
 #endif