Hook up dependency overrides to the rest of pub.

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

Index: sdk/lib/_internal/pub/lib/src/validator/dependency_override.dart
diff --git a/sdk/lib/_internal/pub/lib/src/validator/dependency_override.dart b/sdk/lib/_internal/pub/lib/src/validator/dependency_override.dart
new file mode 100644
index 0000000000000000000000000000000000000000..a5ce299872d03b0fa383072af5ba94e2aca6bc45
--- /dev/null
+++ b/sdk/lib/_internal/pub/lib/src/validator/dependency_override.dart
@@ -0,0 +1,28 @@
+// 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();
+  }
+}