fix assert message tests in dart2js

pkg/compiler/lib/src/diagnostics/messages.dart

 // Copyright (c) 2011, 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.
 
 /**
  * The messages in this file should meet the following guide lines:
  *
  * 1. The message should be a complete sentence starting with an uppercase
  * letter, and ending with a period.
  *
@@ skipping 172 matching lines @@
   EMPTY_CATCH_DECLARATION,
   EMPTY_ENUM_DECLARATION,
   EMPTY_NAMED_PARAMETER_LIST,
   EMPTY_OPTIONAL_PARAMETER_LIST,
   EMPTY_HIDE,
   EMPTY_SHOW,
   EQUAL_MAP_ENTRY_KEY,
   EXISTING_DEFINITION,
   EXISTING_LABEL,
   EXPECTED_IDENTIFIER_NOT_RESERVED_WORD,
-  EXPERIMENTAL_ASSERT_MESSAGE,
   EXPONENT_MISSING,
   EXPORT_BEFORE_PARTS,
   EXTERNAL_WITH_BODY,
   EXTRA_CATCH_DECLARATION,
   EXTRA_FORMALS,
   EXTRANEOUS_MODIFIER,
   EXTRANEOUS_MODIFIER_REPLACE,
   FACTORY_REDIRECTION_IN_NON_FACTORY,
   FINAL_FUNCTION_TYPE_PARAMETER,
   FINAL_WITHOUT_INITIALIZER,
@@ skipping 3447 matching lines @@
 @JS()
 external foo() {}
 main() => foo();
 """
           ]),
 
       //////////////////////////////////////////////////////////////////////////////
       // Patch errors end.
       //////////////////////////////////////////////////////////////////////////////
 
-      MessageKind.EXPERIMENTAL_ASSERT_MESSAGE: const MessageTemplate(
-          MessageKind.EXPERIMENTAL_ASSERT_MESSAGE,
-          "Experimental language feature 'assertion with message'"
-          " is not supported.",
-          howToFix:
-              "Use option '--assert-message' to use assertions with messages.",
-          examples: const [
-            r'''
-main() {
-  int n = -7;
-  assert(n > 0, 'must be positive: $n');
-}
-'''
-          ]),
-
       MessageKind.IMPORT_EXPERIMENTAL_MIRRORS: const MessageTemplate(
           MessageKind.IMPORT_EXPERIMENTAL_MIRRORS,
           r'''
 
 ****************************************************************
 * WARNING: dart:mirrors support in dart2js is experimental,
 *          and not recommended.
 *          This implementation of mirrors is incomplete,
 *          and often greatly increases the size of the generated
 *          JavaScript code.
@@ skipping 135 matching lines @@
   static String convertToString(value) {
     if (value is ErrorToken) {
       // Shouldn't happen.
       return value.assertionMessage;
     } else if (value is Token) {
       value = value.value;
     }
     return '$value';
   }
 }