Revert "Compute correct constructor type for synthesized mixin constructors."

tests/compiler/dart2js/mixin_language_test.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.
-
-// Test that dart2js produces the expected static type warnings for these
-// language tests. This ensures that the analyzer and dart2js agrees on the
-// tests.
-
-import 'warnings_checker.dart';
-
-/// Map from test files to a map of their expected status. If the status map is
-/// `null` no warnings must be missing or unexpected, otherwise the status map
-/// can contain a list of line numbers for keys 'missing' and 'unexpected' for
-/// the warnings of each category.
-const Map<String, dynamic> TESTS = const {
-    'language/typevariable_substitution2_test.dart': null,
-};
-
-void main() {
-  checkWarnings(TESTS);
-}