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

tests/language/typevariable_substitution2_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 mixins don't interfere with type variable substitution.
-
-import 'checked_mode_helper.dart';
-
-class B<T> {
-  B(T x);
-}
-
-class M {}
-
-class A<T> extends B<T> with M {
-  A(T x) : super(x);  // This line must be warning free.
-}
-
-class C<T> = B<T> with M;
-
-
-main() {
-  new A(null);
-  new C<String>(''); /// 01: ok
-  checkDynamicTypeError(() => new C<String>(0)); /// 02: static type warning
-}