Re-land "Format all multitests"

tests/language/const_constructor_mixin_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.
 
-class Mixin {
-}
+class Mixin {}
 
 class A {
   const A(foo);
 }
 
 class B extends A
     with Mixin // //# 01: compile-time error
-    {
+{
   const B(foo) : super(foo);
 }
 
 main() {
   var a = const B(42);
 }