[strong] Strong classes can't extend null

src/compiler/ast-graph-builder.cc

 // Copyright 2014 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.
 
 #include "src/compiler/ast-graph-builder.h"
 
 #include "src/compiler.h"
 #include "src/compiler/ast-loop-assignment-analyzer.h"
 #include "src/compiler/control-builders.h"
 #include "src/compiler/js-type-feedback.h"
@@ skipping 1544 matching lines @@
   VisitForValueOrTheHole(expr->extends());
   VisitForValue(expr->constructor());
 
   // Create node to instantiate a new class.
   Node* constructor = environment()->Pop();
   Node* extends = environment()->Pop();
   Node* name = environment()->Pop();
   Node* script = jsgraph()->Constant(info()->script());
   Node* start = jsgraph()->Constant(expr->start_position());
   Node* end = jsgraph()->Constant(expr->end_position());
-  const Operator* opc = javascript()->CallRuntime(Runtime::kDefineClass, 6);
+  const Operator* opc = javascript()->CallRuntime(
+      is_strong(language_mode()) ? Runtime::kDefineClassStrong
+                                 : Runtime::kDefineClass,
+      6);
   Node* literal = NewNode(opc, name, extends, constructor, script, start, end);
   PrepareFrameState(literal, expr->CreateLiteralId(),
                     OutputFrameStateCombine::Push());
 
   // The prototype is ensured to exist by Runtime_DefineClass. No access check
   // is needed here since the constructor is created by the class literal.
   Node* proto =
       BuildLoadObjectField(literal, JSFunction::kPrototypeOrInitialMapOffset);
 
   // The class literal and the prototype are both expected on the operand stack
@@ skipping 2659 matching lines @@
     // Phi does not exist yet, introduce one.
     value = NewPhi(inputs, value, control);
     value->ReplaceInput(inputs - 1, other);
   }
   return value;
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8