Add optional message to assert in Dart2js.

pkg/compiler/lib/src/common/backend_api.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.backend_api;
 
 import 'dart:async' show Future;
 
 import '../compiler.dart' show
     Compiler;
@@ skipping 149 matching lines @@
   /// Register an is check to the backend.
   void registerIsCheckForCodegen(DartType type,
                                  Enqueuer enqueuer,
                                  Registry registry) {}
 
   /// Register a runtime type variable bound tests between [typeArgument] and
   /// [bound].
   void registerTypeVariableBoundsSubtypeCheck(DartType typeArgument,
                                               DartType bound) {}
 
-  /// Returns `true` if [element] represent the assert function.
-  bool isAssertMethod(Element element) => false;
-
   /**
    * Call this to register that an instantiated generic class has a call
    * method.
    */
   void registerCallMethodWithFreeTypeVariables(
       Element callMethod,
       Enqueuer enqueuer,
       Registry registry) {}
 
   /**
@@ skipping 172 matching lines @@
 
   void forgetElement(Element element) {}
 
   void registerMainHasArguments(Enqueuer enqueuer) {}
 
   void registerAsyncMarker(FunctionElement element,
                            Enqueuer enqueuer,
                            Registry registry) {}
 }