Reland of Thread maybe-assigned through the bytecodes.

src/interpreter/bytecode-array-builder.h

Index: src/interpreter/bytecode-array-builder.h
diff --git a/src/interpreter/bytecode-array-builder.h b/src/interpreter/bytecode-array-builder.h
index 2b25400cabd3b9b2d2f03fbd3cc5f501ea6ab254..e734287e2d1297a0d6435906b502b2aeae945943 100644
--- a/src/interpreter/bytecode-array-builder.h
+++ b/src/interpreter/bytecode-array-builder.h
@@ -92,8 +92,10 @@
 
   // 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|.