[strong] weak classes can't inherit from strong ones

test/mjsunit/strong/class-weak-extend.js

+// 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.
+
+// Flags: --strong-mode --allow-natives-syntax
+
+function getStrongClass() {
+  "use strong";
+  return (class {});
+}
+
+function weakClass() {
+  "use strict";
+  class Weak extends getStrongClass() {}
+}
+
+function strongClass() {
+  "use strong";
+  class Strong extends getStrongClass() {}
+}
+
+assertThrows(weakClass, TypeError);
+%OptimizeFunctionOnNextCall(weakClass);
+assertThrows(weakClass, TypeError);
+
+assertDoesNotThrow(strongClass);
+%OptimizeFunctionOnNextCall(strongClass);
+assertDoesNotThrow(strongClass);