Fix Win64 build of Dart VM.

runtime/vm/assembler_x64.h

Index: runtime/vm/assembler_x64.h
diff --git a/runtime/vm/assembler_x64.h b/runtime/vm/assembler_x64.h
index 84ba32016be333eca0099ded63782266ddea832d..5a141fd08a131772ac8fd54122720f617c0288ac 100644
--- a/runtime/vm/assembler_x64.h
+++ b/runtime/vm/assembler_x64.h
@@ -714,6 +714,9 @@ class Assembler : public ValueObject {
     cmpxchgl(address, reg);
   }
 
+  void PushRegisters(intptr_t cpu_register_set, intptr_t xmm_register_set);
+  void PopRegisters(intptr_t cpu_register_set, intptr_t xmm_register_set);
+
   void EnterFrame(intptr_t frame_space);
   void LeaveFrame();
   void ReserveAlignedFrameSpace(intptr_t frame_space);
@@ -726,6 +729,11 @@ class Assembler : public ValueObject {
 
   void CallRuntime(const RuntimeEntry& entry, intptr_t argument_count);
 
+  // Call runtime function. Reserves shadow space on the stack before calling
+  // if platform ABI requires that. Does not restore RSP after the call itself.
+  void CallCFunction(const ExternalLabel* label);
+  void CallCFunction(Register reg);
+
   /*
    * Loading and comparing classes of objects.
    */