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

src/messages.h

 // Copyright 2006-2008 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.
 
 // The infrastructure used for (localized) message reporting in V8.
 //
 // Note: there's a big unresolved issue about ownership of the data
 // structures used by this framework.
 
 #ifndef V8_MESSAGES_H_
@@ skipping 207 matching lines @@
   T(StrongDeleteProperty,                                                      \
     "Deleting property '%' of strong object '%' is deprecated")                \
   T(StrongExtendNull, "In strong mode, classes extending null are deprecated") \
   T(StrongImplicitConversion,                                                  \
     "In strong mode, implicit conversions are deprecated")                     \
   T(StrongRedefineDisallowed,                                                  \
     "On strong object %, redefining writable, non-configurable property '%' "  \
     "to be non-writable is deprecated")                                        \
   T(StrongSetProto,                                                            \
     "On strong object %, redefining the internal prototype is deprecated")     \
+  T(StrongWeakExtend, "Non-strong class % cannot extend a strong object")      \
   T(SymbolKeyFor, "% is not a symbol")                                         \
   T(SymbolToNumber, "Cannot convert a Symbol value to a number")               \
   T(SymbolToString, "Cannot convert a Symbol value to a string")               \
   T(SimdToNumber, "Cannot convert a SIMD value to a number")                   \
   T(UndefinedOrNullToObject, "Cannot convert undefined or null to object")     \
   T(ValueAndAccessor,                                                          \
     "Invalid property.  A property cannot both have accessors and be "         \
     "writable or have a value, %")                                             \
   T(VarRedeclaration, "Identifier '%' has already been declared")              \
   T(WithExpression, "% has no properties")                                     \
@@ skipping 247 matching lines @@
 
   static MUST_USE_RESULT MaybeHandle<String> GetStringifiedProperty(
       Isolate* isolate, LookupIterator* property_lookup,
       Handle<String> default_value);
 
   List<Handle<JSObject> > visited_;
 };
 } }  // namespace v8::internal
 
 #endif  // V8_MESSAGES_H_