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

runtime/vm/atomic.h

Index: runtime/vm/atomic.h
diff --git a/runtime/vm/atomic.h b/runtime/vm/atomic.h
index 809548a3c38933453dc7c5d92db2d06076cbe89d..59a45ace74cc42b1d22c7bc6ee4aad92cb9542c5 100644
--- a/runtime/vm/atomic.h
+++ b/runtime/vm/atomic.h
@@ -18,15 +18,17 @@ class AtomicOperations : public AllStatic {
   // Returns the original value at p.
   //
   // NOTE: Not to be used for any atomic operations involving memory locations
-  // that are accessed by generated code
+  // that are accessed by generated code.
   static uintptr_t FetchAndIncrement(uintptr_t* p);
+  static uintptr_t FetchAndIncrementBy(intptr_t* p, intptr_t value);
 
   // Atomically fetch the value at p and decrement the value at p.
   // Returns the original value at p.
   //
   // NOTE: Not to be used for any atomic operations involving memory locations
-  // that are accessed by generated code
+  // that are accessed by generated code.
   static uintptr_t FetchAndDecrement(uintptr_t* p);
+  static uintptr_t FetchAndDecrementBy(intptr_t* p, intptr_t value);
 
   // Atomically compare *ptr to old_value, and if equal, store new_value.
   // Returns the original value at ptr.