Reland of Thread maybe-assigned through the bytecodes.

src/interpreter/bytecode-array-builder.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/interpreter/bytecode-array-builder.h"
 
 #include "src/globals.h"
 #include "src/interpreter/bytecode-array-writer.h"
 #include "src/interpreter/bytecode-dead-code-optimizer.h"
 #include "src/interpreter/bytecode-label.h"
@@ skipping 459 matching lines @@
   size_t name_index = GetConstantPoolEntry(name);
   if (language_mode == SLOPPY) {
     OutputStaGlobalSloppy(name_index, feedback_slot);
   } else {
     DCHECK_EQ(language_mode, STRICT);
     OutputStaGlobalStrict(name_index, feedback_slot);
   }
   return *this;
 }
 
-BytecodeArrayBuilder& BytecodeArrayBuilder::LoadContextSlot(Register context,
-                                                            int slot_index,
-                                                            int depth) {
+BytecodeArrayBuilder& BytecodeArrayBuilder::LoadContextSlot(
+    Register context, int slot_index, int depth,
+    ContextSlotMutability mutability) {
   if (context.is_current_context() && depth == 0) {
-    OutputLdaCurrentContextSlot(slot_index);
+    if (mutability == kImmutableSlot) {
+      OutputLdaImmutableCurrentContextSlot(slot_index);
+    } else {
+      DCHECK_EQ(kMutableSlot, mutability);
+      OutputLdaCurrentContextSlot(slot_index);
+    }
+  } else if (mutability == kImmutableSlot) {
+    OutputLdaImmutableContextSlot(context, slot_index, depth);
   } else {
+    DCHECK_EQ(mutability, kMutableSlot);
     OutputLdaContextSlot(context, slot_index, depth);
   }
   return *this;
 }
 
 BytecodeArrayBuilder& BytecodeArrayBuilder::StoreContextSlot(Register context,
                                                              int slot_index,
                                                              int depth) {
   if (context.is_current_context() && depth == 0) {
     OutputStaCurrentContextSlot(slot_index);
@@ skipping 587 matching lines @@
     RegisterList reg_list) {
   DCHECK(RegisterListIsValid(reg_list));
   if (register_optimizer_)
     register_optimizer_->PrepareOutputRegisterList(reg_list);
   return static_cast<uint32_t>(reg_list.first_register().ToOperand());
 }
 
 }  // namespace interpreter
 }  // namespace internal
 }  // namespace v8