fix assert message tests in dart2js

pkg/compiler/lib/src/options.dart

 // Copyright (c) 2016, 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.src.options;
 
 import '../compiler.dart' show PackagesDiscoveryProvider;
 import 'commandline_options.dart' show Flags;
 
 /// Options used for controlling diagnostic messages.
@@ skipping 113 matching lines @@
   final bool disableTypeInference;
 
   /// Whether to emit a .json file with a summary of the information used by the
   /// compiler during optimization. This includes resolution details,
   /// dependencies between elements, results of type inference, and the output
   /// code for each function.
   final bool dumpInfo;
 
   /// Whether we allow passing an extra argument to `assert`, containing a
   /// reason for why an assertion fails. (experimental)
+  ///
+  /// This is only included so that tests can pass the --assert-message flag
+  /// without causing dart2js to crash. The flag has no effect.
   final bool enableAssertMessage;
 
   /// Whether the user specified a flag to allow the use of dart:mirrors. This
   /// silences a warning produced by the compiler.
   final bool enableExperimentalMirrors;
 
   /// Whether to enable minification
   // TODO(sigmund): rename to minify
   final bool enableMinification;
 
@@ skipping 554 matching lines @@
 }
 
 /// Locations of the platform descriptor files relative to the library root.
 const String _clientPlatform = "lib/dart_client.platform";
 const String _serverPlatform = "lib/dart_server.platform";
 const String _sharedPlatform = "lib/dart_shared.platform";
 
 const String _UNDETERMINED_BUILD_ID = "build number could not be determined";
 const bool _forceIncrementalSupport =
     const bool.fromEnvironment('DART2JS_EXPERIMENTAL_INCREMENTAL_SUPPORT');