Reland of Thread maybe-assigned through the bytecodes.

src/interpreter/bytecode-array-builder.h

 // 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.
 
 #ifndef V8_INTERPRETER_BYTECODE_ARRAY_BUILDER_H_
 #define V8_INTERPRETER_BYTECODE_ARRAY_BUILDER_H_
 
 #include "src/ast/ast.h"
 #include "src/base/compiler-specific.h"
 #include "src/globals.h"
@@ skipping 74 matching lines @@
 
   // Global loads to the accumulator and stores from the accumulator.
   BytecodeArrayBuilder& LoadGlobal(const Handle<String> name, int feedback_slot,
                                    TypeofMode typeof_mode);
   BytecodeArrayBuilder& StoreGlobal(const Handle<String> name,
                                     int feedback_slot,
                                     LanguageMode language_mode);
 
   // Load the object at |slot_index| at |depth| in the context chain starting
   // with |context| into the accumulator.
+  enum ContextSlotMutability { kImmutableSlot, kMutableSlot };
   BytecodeArrayBuilder& LoadContextSlot(Register context, int slot_index,
-                                        int depth);
+                                        int depth,
+                                        ContextSlotMutability immutable);
 
   // Stores the object in the accumulator into |slot_index| at |depth| in the
   // context chain starting with |context|.
   BytecodeArrayBuilder& StoreContextSlot(Register context, int slot_index,
                                          int depth);
 
   // Load from a module variable into the accumulator. |depth| is the depth of
   // the current context relative to the module context.
   BytecodeArrayBuilder& LoadModuleVariable(int cell_index, int depth);
 
@@ skipping 342 matching lines @@
   static int const kNoFeedbackSlot = 0;
 
   DISALLOW_COPY_AND_ASSIGN(BytecodeArrayBuilder);
 };
 
 }  // namespace interpreter
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_INTERPRETER_BYTECODE_ARRAY_BUILDER_H_