fix assert with message

pkg/compiler/lib/src/dart2js.dart

 // Copyright (c) 2013, 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.cmdline;
 
 import 'dart:async' show EventSink, Future;
 import 'dart:convert' show UTF8, LineSplitter;
 import 'dart:io' show exit, File, FileMode, Platform, stdin, stderr;
 
@@ skipping 359 matching lines @@
     new OptionHandler(Flags.disableInlining, implyCompilation),
     new OptionHandler(Flags.disableTypeInference, implyCompilation),
     new OptionHandler(Flags.terse, passThrough),
     new OptionHandler('--deferred-map=.+', implyCompilation),
     new OptionHandler(Flags.dumpInfo, implyCompilation),
     new OptionHandler(
         '--disallow-unsafe-eval', (_) => hasDisallowUnsafeEval = true),
     new OptionHandler(Option.showPackageWarnings, passThrough),
     new OptionHandler(Flags.useContentSecurityPolicy, passThrough),
     new OptionHandler(Flags.enableExperimentalMirrors, passThrough),
-    new OptionHandler(Flags.enableAssertMessage, passThrough),
 
     // TODO(floitsch): remove conditional directives flag.
     // We don't provide the info-message yet, since we haven't publicly
     // launched the feature yet.
     new OptionHandler(Flags.conditionalDirectives, (_) {}),
     new OptionHandler('--enable-async', (_) {
       hints.add("Option '--enable-async' is no longer needed. "
           "Async-await is supported by default.");
     }),
     new OptionHandler('--enable-null-aware-operators', (_) {
@@ skipping 642 matching lines @@
   @override
   void close() {
     // Do nothing.
   }
 
   @override
   void addError(errorEvent, [StackTrace stackTrace]) {
     // Ignore
   }
 }