Add unboxed Mint for X64

runtime/vm/flow_graph_compiler_x64.cc

Index: runtime/vm/flow_graph_compiler_x64.cc
diff --git a/runtime/vm/flow_graph_compiler_x64.cc b/runtime/vm/flow_graph_compiler_x64.cc
index 05ee329b5987077051276364d187322762ea6889..790fea4c94c1faf338585b614f90dee51881a697 100644
--- a/runtime/vm/flow_graph_compiler_x64.cc
+++ b/runtime/vm/flow_graph_compiler_x64.cc
@@ -22,6 +22,7 @@
 namespace dart {
 
 DEFINE_FLAG(bool, trap_on_deoptimization, false, "Trap on deoptimization.");
+DEFINE_FLAG(bool, unbox_mints, true, "Optimize 64-bit integer arithmetic.");
 DECLARE_FLAG(bool, enable_type_checks);
 DECLARE_FLAG(bool, enable_simd_inline);
 
@@ -42,7 +43,7 @@ bool FlowGraphCompiler::SupportsUnboxedDoubles() {
 
 
 bool FlowGraphCompiler::SupportsUnboxedMints() {
-  return false;
+  return FLAG_unbox_mints;
 }