Revert "Use native async/await support in pub."

sdk/lib/_internal/pub_generated/lib/src/validator/dependency_override.dart

 // Copyright (c) 2012, 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.
 
 library pub.validator.dependency_override;
 
 import 'dart:async';
 
 import '../entrypoint.dart';
 import '../validator.dart';
 
 /// A validator that validates a package's dependencies overrides (or the
 /// absence thereof).
 class DependencyOverrideValidator extends Validator {
   DependencyOverrideValidator(Entrypoint entrypoint)
-    : super(entrypoint);
+      : super(entrypoint);
 
   Future validate() {
     if (entrypoint.root.dependencyOverrides.isNotEmpty) {
       errors.add(
           'Your pubspec.yaml must not have a "dependency_overrides" field.\n'
-          'This ensures you test your package against the same versions of '
-              'its dependencies\n'
-          'that users will have when they use it.');
+              'This ensures you test your package against the same versions of '
+              'its dependencies\n' 'that users will have when they use it.');
     }
     return new Future.value();
   }
 }