dart2js: run dartfmt on tests

tests/compiler/dart2js_extra/null_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.
 
 import "package:expect/expect.dart";
 
 null1() {
-  return;  // implicit null;
+  return; // implicit null;
 }
 
 null2() {
   // Implicit null return.
 }
 
 null3() {
-  var x;  // Implicit null value.
+  var x; // Implicit null value.
   return x;
 }
 
 void main() {
   Expect.equals(null, null1());
   Expect.equals(null, null2());
   Expect.equals(null, null3());
 }