Reland of Thread maybe-assigned through the bytecodes.

test/mjsunit/immutable-context-slot-inlining.js

+// Copyright 2017 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.
+
+// Flags: --allow-natives-syntax
+
+function h(g) {
+  return g();
+}
+
+function f() {
+  var g;
+  for (var i = 0; i < 10; i++) {
+    var y = i;
+    if (i === 5) {
+      g = function() {
+        return y;
+      };
+      assertEquals(5, h(g));
+      assertEquals(5, h(g));
+      %OptimizeFunctionOnNextCall(h);
+      assertEquals(5, h(g));
+    }
+  }
+  return g;
+}
+
+var myg = f();
+
+assertEquals(9, h(myg));