Thread maybe-assigned through the bytecodes.

src/interpreter/bytecode-generator.cc

Index: src/interpreter/bytecode-generator.cc
diff --git a/src/interpreter/bytecode-generator.cc b/src/interpreter/bytecode-generator.cc
index c28d4687d8a8cf5b742626739d8cbeb50ee9dc54..75497cf2c16738f53ac9c24a8b01c94ea90ec75f 100644
--- a/src/interpreter/bytecode-generator.cc
+++ b/src/interpreter/bytecode-generator.cc
@@ -1942,7 +1942,13 @@ void BytecodeGenerator::BuildVariableLoad(Variable* variable,
         context_reg = execution_context()->reg();
       }
 
-      builder()->LoadContextSlot(context_reg, variable->index(), depth);
+      BytecodeArrayBuilder::ContextSlotMutability immutable =
+          (variable->maybe_assigned() == kNotAssigned)
+              ? BytecodeArrayBuilder::kImmutableSlot
+              : BytecodeArrayBuilder::kMutableSlot;
+
+      builder()->LoadContextSlot(context_reg, variable->index(), depth,
+                                 immutable);
       if (hole_check_mode == HoleCheckMode::kRequired) {
         BuildThrowIfHole(variable->name());
       }
@@ -2106,7 +2112,8 @@ void BytecodeGenerator::BuildVariableAssignment(Variable* variable,
         Register value_temp = register_allocator()->NewRegister();
         builder()
             ->StoreAccumulatorInRegister(value_temp)
-            .LoadContextSlot(context_reg, variable->index(), depth);
+            .LoadContextSlot(context_reg, variable->index(), depth,
+                             BytecodeArrayBuilder::kMutableSlot);
 
         BuildHoleCheckForVariableAssignment(variable, op);
         builder()->LoadAccumulatorWithRegister(value_temp);
@@ -2737,9 +2744,11 @@ void BytecodeGenerator::VisitDelete(UnaryOperation* expr) {
         Register global_object = register_allocator()->NewRegister();
         builder()
             ->LoadContextSlot(execution_context()->reg(),
-                              Context::NATIVE_CONTEXT_INDEX, 0)
+                              Context::NATIVE_CONTEXT_INDEX, 0,
+                              BytecodeArrayBuilder::kMutableSlot)
             .StoreAccumulatorInRegister(native_context)
-            .LoadContextSlot(native_context, Context::EXTENSION_INDEX, 0)
+            .LoadContextSlot(native_context, Context::EXTENSION_INDEX, 0,
+                             BytecodeArrayBuilder::kMutableSlot)
             .StoreAccumulatorInRegister(global_object)
             .LoadLiteral(variable->name())
             .Delete(global_object, language_mode());
@@ -3251,15 +3260,18 @@ void BytecodeGenerator::VisitFunctionClosureForContext() {
     Register native_context = register_allocator()->NewRegister();
     builder()
         ->LoadContextSlot(execution_context()->reg(),
-                          Context::NATIVE_CONTEXT_INDEX, 0)
+                          Context::NATIVE_CONTEXT_INDEX, 0,
+                          BytecodeArrayBuilder::kMutableSlot)
         .StoreAccumulatorInRegister(native_context)
-        .LoadContextSlot(native_context, Context::CLOSURE_INDEX, 0);
+        .LoadContextSlot(native_context, Context::CLOSURE_INDEX, 0,
+                         BytecodeArrayBuilder::kMutableSlot);
   } else if (closure_scope->is_eval_scope()) {
     // Contexts created by a call to eval have the same closure as the
     // context calling eval, not the anonymous closure containing the eval
     // code. Fetch it from the context.
     builder()->LoadContextSlot(execution_context()->reg(),
-                               Context::CLOSURE_INDEX, 0);
+                               Context::CLOSURE_INDEX, 0,
+                               BytecodeArrayBuilder::kMutableSlot);
   } else {
     DCHECK(closure_scope->is_function_scope() ||
            closure_scope->is_module_scope());