Un revert the revert

runtime/vm/assembler_x64.cc

 // Copyright (c) 2011, 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.
 
 #include "vm/globals.h"
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/assembler.h"
 #include "vm/heap.h"
 #include "vm/memory_region.h"
@@ skipping 1248 matching lines @@
     value = -value;
     if (value == 1) {
       decq(reg);
     } else if (value != 0) {
       subq(reg, Immediate(value));
     }
   }
 }
 
 
+void Assembler::Drop(intptr_t stack_elements) {
+  ASSERT(stack_elements >= 0);
+  if (stack_elements > 0) {
+    // TODO(fschneider): When optimizing for code size, we could
+    // consider using pop for stack_elements < 4 instead.
+    addq(RSP, Immediate(stack_elements * kWordSize));
+  }
+}
+
+
 void Assembler::LoadObject(Register dst, const Object& object) {
   if (object.IsSmi()) {
     movq(dst, Immediate(reinterpret_cast<int64_t>(object.raw())));
   } else {
     ASSERT(object.IsZoneHandle());
     AssemblerBuffer::EnsureCapacity ensured(&buffer_);
     EmitRegisterREX(dst, REX_W);
     EmitUint8(0xB8 | (dst & 7));
     buffer_.EmitObject(object);
   }
@@ skipping 223 matching lines @@
   } else {
     EmitRegisterREX(operand, REX_NONE);
   }
   EmitUint8(0xD3);
   EmitOperand(rm, Operand(operand));
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64