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

test/mjsunit/strong/literals.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
 // Flags: --harmony-arrow-functions --harmony-rest-parameters
 // Flags: --harmony-destructuring --harmony-spread-arrays
 
 'use strict';
 
@@ skipping 267 matching lines @@
 (function WeakClassLiterals() {
   function assertWeakClass(x) {
     assertFalse(%IsStrong(x));
     assertFalse(%IsStrong(x.prototype));
     assertFalse(%IsStrong(new x));
   }
   class C {};
   class D extends C {};
   class E extends Object {};
   // class F extends null {};
-  const S = (() => {'use strong'; return class {}})();
-  class G extends S {};
   assertWeakClass(C);
   assertWeakClass(D);
   assertWeakClass(E);
   // assertWeakClass(F);
-  assertWeakClass(G);
   assertWeakClass(class {});
   assertWeakClass(class extends Object {});
   // assertWeakClass(class extends null {});
   assertWeakClass(class extends C {});
-  assertWeakClass(class extends S {});
   assertWeakClass(class extends class {} {});
   assertWeakClass(class C {});
   assertWeakClass(class D extends Object {});
   // assertWeakClass(class D extends null {});
   assertWeakClass(class D extends C {});
-  assertWeakClass(class D extends S {});
   assertWeakClass(class D extends class {} {});
 })();
 
 (function StrongClassLiterals() {
   'use strong';
   function assertStrongClass(x) {
     assertTrue(%IsStrong(x));
     assertTrue(%IsStrong(x.prototype));
     // TODO(rossberg): strongify class instance
     // assertTrue(%IsStrong(new x));
@@ skipping 27 matching lines @@
 })();
 
 (function StrongRegExpLiterals() {
   'use strong';
   function assertStrongRegExp(x) {
     // TODO(rossberg): strongify regexps
     // assertTrue(%IsStrong(x));
   }
   assertStrongRegExp(/abc/);
 })();