Loosen dependency override validation.

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.
 
 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) {
+    var overridden =
+        entrypoint.root.dependencyOverrides.map((dep) => dep.name).toSet();
+    var dev = entrypoint.root.devDependencies.map((dep) => dep.name).toSet();
+    if (overridden.difference(dev).isNotEmpty) {
       errors.add(
-          'Your pubspec.yaml must not have a "dependency_overrides" field.\n'
+          'Your pubspec.yaml must not override non-dev dependencies.\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();
   }
 }