Setup R10/EDX to be valid Oops before stub calls

runtime/vm/intermediate_language_x64.cc

Index: runtime/vm/intermediate_language_x64.cc
diff --git a/runtime/vm/intermediate_language_x64.cc b/runtime/vm/intermediate_language_x64.cc
index ffc7abeca33b8ec1c27713771b417823148d4586..470e30ed904f487dc9c82ee2721d100376c4a11d 100644
--- a/runtime/vm/intermediate_language_x64.cc
+++ b/runtime/vm/intermediate_language_x64.cc
@@ -5825,6 +5825,19 @@ void AllocateObjectInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   __ Drop(ArgumentCount());  // Discard arguments.
 }
 
+
+void DebugStepCheckInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
+  ASSERT(!compiler->is_optimizing());
+  const ExternalLabel label(StubCode::DebugStepCheckEntryPoint());
+  __ movq(R10, Immediate(0));
+  __ movq(RBX, Immediate(0));
+  compiler->GenerateCall(token_pos(), &label, stub_kind_, locs());
+#if defined(DEBUG)
+  __ movq(R10, Immediate(1));  // Garbage that looks like an object.
+  __ movq(RBX, Immediate(1));
+#endif
+}
+
 }  // namespace dart
 
 #undef __