Reland of Thread maybe-assigned through the bytecodes.

test/unittests/interpreter/bytecode-array-builder-unittest.cc

Index: test/unittests/interpreter/bytecode-array-builder-unittest.cc
diff --git a/test/unittests/interpreter/bytecode-array-builder-unittest.cc b/test/unittests/interpreter/bytecode-array-builder-unittest.cc
index 98b90bb7e24b853e033082555b7380cddc22109b..2ef43c4c33abda256338364e92c18f0dbc3d3c73 100644
--- a/test/unittests/interpreter/bytecode-array-builder-unittest.cc
+++ b/test/unittests/interpreter/bytecode-array-builder-unittest.cc
@@ -86,12 +86,19 @@
   // Emit context operations.
   builder.PushContext(reg)
       .PopContext(reg)
-      .LoadContextSlot(reg, 1, 0)
-      .StoreContextSlot(reg, 1, 0);
+      .LoadContextSlot(reg, 1, 0, BytecodeArrayBuilder::kMutableSlot)
+      .StoreContextSlot(reg, 1, 0)
+      .LoadContextSlot(reg, 2, 0, BytecodeArrayBuilder::kImmutableSlot)
+      .StoreContextSlot(reg, 3, 0);
 
   // Emit context operations which operate on the local context.
-  builder.LoadContextSlot(Register::current_context(), 1, 0)
-      .StoreContextSlot(Register::current_context(), 1, 0);
+  builder
+      .LoadContextSlot(Register::current_context(), 1, 0,
+                       BytecodeArrayBuilder::kMutableSlot)
+      .StoreContextSlot(Register::current_context(), 1, 0)
+      .LoadContextSlot(Register::current_context(), 2, 0,
+                       BytecodeArrayBuilder::kImmutableSlot)
+      .StoreContextSlot(Register::current_context(), 3, 0);
 
   // Emit load / store property operations.
   builder.LoadNamedProperty(reg, name, 0)
@@ -327,7 +334,8 @@
                                      DataPropertyInLiteralFlag::kNoFlags, 0);
 
   // Emit wide context operations.
-  builder.LoadContextSlot(reg, 1024, 0).StoreContextSlot(reg, 1024, 0);
+  builder.LoadContextSlot(reg, 1024, 0, BytecodeArrayBuilder::kMutableSlot)
+      .StoreContextSlot(reg, 1024, 0);
 
   // Emit wide load / store lookup slots.
   builder.LoadLookupSlot(wide_name, TypeofMode::NOT_INSIDE_TYPEOF)