Format all tests.

tests/language/prefix15_negative_test.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.
 //
 // Unresolved symbols should be reported as an error.
 
 library Prefix15NegativeTest.dart;
+
 import "library12.dart" as lib12;
 
 class myClass implements lib12.Library13 {
   myClass(int this.fld) : super(0);
   int fld;
 }
 
 main() {
   new myClass(1);
 }