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

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 8e07cd3d20b2330f60d2cc3a8d9e009af41843b6..4172ec443c4a1d1a190252872f1b83b2f3647941 100644
--- a/runtime/vm/atomic_win.h
+++ b/runtime/vm/atomic_win.h
@@ -28,6 +28,22 @@ inline uintptr_t AtomicOperations::FetchAndIncrement(uintptr_t* p) {
 }
 
 
+inline intptr_t AtomicOperations::FetchAndIncrementBy(intptr_t* p,
+                                                      intptr_t value) {
+#if defined(HOST_ARCH_X64)
+  return static_cast<intptr_t>(
+      InterlockedAdd64(reinterpret_cast<LONGLONG*>(p),
+                       static_cast<LONGLONG>(value))) - value;
+#elif defined(HOST_ARCH_IA32)
+  return static_cast<intptr_t>(
+      InterlockedAdd(reinterpret_cast<LONG*>(p),
+                     static_cast<LONGLONG>(value))) - value;
+#else
+#error Unsupported host architecture.
+#endif
+}
+
+
 inline uintptr_t AtomicOperations::FetchAndDecrement(uintptr_t* p) {
 #if defined(HOST_ARCH_X64)
   return static_cast<uintptr_t>(
@@ -41,6 +57,22 @@ inline uintptr_t AtomicOperations::FetchAndDecrement(uintptr_t* p) {
 }
 
 
+inline intptr_t AtomicOperations::FetchAndDecrementBy(intptr_t* p,
+                                                      intptr_t value) {
+#if defined(HOST_ARCH_X64)
+  return static_cast<intptr_t>(
+      InterlockedAdd64(reinterpret_cast<LONGLONG*>(p),
+                       static_cast<LONGLONG>(-value))) + value;
+#elif defined(HOST_ARCH_IA32)
+  return static_cast<intptr_t>(
+      InterlockedAdd(reinterpret_cast<LONG*>(p),
+                     static_cast<LONGLONG>(-value))) + value;
+#else
+#error Unsupported host architecture.
+#endif
+}
+
+
 #if !defined(USING_SIMULATOR)
 inline uword AtomicOperations::CompareAndSwapWord(uword* ptr,
                                                   uword old_value,