Format all tests.

tests/language/private_mixin_exception_throw_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.
 
 // Regression test for http://dartbug.com/11637
 
 class _C {}
+
 class _E {
   throwIt() => throw "it";
 }
+
 class _F {
   throwIt() => throw "IT";
 }
+
 class _D extends _C with _E, _F {}
 
 main() {
   var d = new _D();
   try {
     d.throwIt();
   } catch (e, s) {
     print("Exception: $e");
     print("Stacktrace:\n$s");
   }
 }