Start pulling pub from its own repo.

sdk/lib/_internal/pub/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);
-
-  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.');
-    }
-    return new Future.value();
-  }
-}