Implement basic support for deferred slow path code with calls that save and restore live registers.

runtime/vm/flow_graph_compiler_x64.h

Index: runtime/vm/flow_graph_compiler_x64.h
diff --git a/runtime/vm/flow_graph_compiler_x64.h b/runtime/vm/flow_graph_compiler_x64.h
index b46a5a4e0571800c8c5a9ee559b6564c0b4e3e97..03f35d4ada6702e512a0b06aae49d854e2adeb5d 100644
--- a/runtime/vm/flow_graph_compiler_x64.h
+++ b/runtime/vm/flow_graph_compiler_x64.h
@@ -191,6 +191,8 @@ class FlowGraphCompiler : public ValueObject {
                       Register reg2 = kNoRegister,
                       Register reg3 = kNoRegister);
 
+  void AddSlowPathCode(SlowPathCode* slow_path);
+
   void FinalizeExceptionHandlers(const Code& code);
   void FinalizePcDescriptors(const Code& code);
   void FinalizeDeoptInfo(const Code& code);
@@ -211,6 +213,9 @@ class FlowGraphCompiler : public ValueObject {
 
   static const int kLocalsOffsetFromFP = (-1 * kWordSize);
 
+  void SaveLiveRegisters(LocationSummary* locs);
+  void RestoreLiveRegisters(LocationSummary* locs);
+
  private:
   friend class DeoptimizationStub;
 
@@ -305,6 +310,7 @@ class FlowGraphCompiler : public ValueObject {
   StackmapTableBuilder* stackmap_table_builder_;
   GrowableArray<BlockInfo*> block_info_;
   GrowableArray<DeoptimizationStub*> deopt_stubs_;
+  GrowableArray<SlowPathCode*> slow_path_code_;
   const GrowableObjectArray& object_table_;
   const bool is_optimizing_;
   const bool is_ssa_;