Un revert the revert

runtime/vm/assembler_x64.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_ASSEMBLER_X64_H_
 #define VM_ASSEMBLER_X64_H_
 
 #ifndef VM_ASSEMBLER_H_
 #error Do not include assembler_x64.h directly; use assembler.h instead.
 #endif
@@ skipping 471 matching lines @@
   void lock_cmpxchgq(const Address& address, Register reg) {
     lock();
     cmpxchgq(address, reg);
   }
 
   /*
    * Macros for High-level operations.
    */
   void AddImmediate(Register reg, const Immediate& imm);
 
+  void Drop(intptr_t stack_elements);
+
   void LoadObject(Register dst, const Object& object);
   void PushObject(const Object& object);
   void CompareObject(Register reg, const Object& object);
   void LoadDoubleConstant(XmmRegister dst, double value);
 
   void StoreIntoObject(Register object,  // Object we are storing into.
                        const FieldAddress& dest,  // Where we are storing into.
                        Register value);  // Value we are storing.
 
   void DoubleNegate(XmmRegister d);
@@ skipping 109 matching lines @@
 }
 
 
 inline void Assembler::EmitOperandSizeOverride() {
   EmitUint8(0x66);
 }
 
 }  // namespace dart
 
 #endif  // VM_ASSEMBLER_X64_H_