Fix exception for assignment to uninitialised const

src/x64/full-codegen-x64.cc

Index: src/x64/full-codegen-x64.cc
diff --git a/src/x64/full-codegen-x64.cc b/src/x64/full-codegen-x64.cc
index 6805b54b907076d849991d3c5af85023e426abd4..c17ed4ec3f60981f7b806c09025306218950ffb8 100644
--- a/src/x64/full-codegen-x64.cc
+++ b/src/x64/full-codegen-x64.cc
@@ -2668,6 +2668,20 @@ void FullCodeGenerator::EmitVariableAssignment(Variable* var,
     __ bind(&assign);
     EmitStoreToStackLocalOrContextSlot(var, location);
 
+  } else if (var->mode() == CONST && op != Token::INIT_CONST) {
+    // Assignment to const variable needs a write barrier.
+    DCHECK(!var->IsLookupSlot());
+    DCHECK(var->IsStackAllocated() || var->IsContextSlot());
+    Label const_error;
+    MemOperand location = VarOperand(var, rcx);
+    __ movp(rdx, location);
+    __ CompareRoot(rdx, Heap::kTheHoleValueRootIndex);
+    __ j(not_equal, &const_error, Label::kNear);
+    __ Push(var->name());
+    __ CallRuntime(Runtime::kThrowReferenceError, 1);
+    __ bind(&const_error);
+    __ CallRuntime(Runtime::kThrowConstAssignError, 0);
+
   } else if (!var->is_const_mode() || op == Token::INIT_CONST) {
     if (var->IsLookupSlot()) {
       // Assignment to var.