Support "pub downgrade".

sdk/lib/_internal/pub/test/downgrade/does_not_show_other_versions_test.dart

Index: sdk/lib/_internal/pub/test/downgrade/does_not_show_other_versions_test.dart
diff --git a/sdk/lib/_internal/pub/test/downgrade/does_not_show_other_versions_test.dart b/sdk/lib/_internal/pub/test/downgrade/does_not_show_other_versions_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..e62bff04a455ff56038eda0b140c6b94af2ffa8f
--- /dev/null
+++ b/sdk/lib/_internal/pub/test/downgrade/does_not_show_other_versions_test.dart
@@ -0,0 +1,34 @@
+// Copyright (c) 2013, 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_tests;
+
+import 'package:scheduled_test/scheduled_test.dart';
+
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+
+main() {
+  initConfig();
+  integration("does not show how many other versions are available", () {
+    servePackages([
+      packageMap("downgraded", "1.0.0"),
+      packageMap("downgraded", "2.0.0"),
+      packageMap("downgraded", "3.0.0-dev")
+    ]);
+
+    d.appDir({
+      "downgraded": "3.0.0-dev"
+    }).create();
+
+    pubGet();
+
+    // Loosen the constraints.
+    d.appDir({
+      "downgraded": ">=2.0.0"
+    }).create();
+
+    pubDowngrade(output: contains("downgraded 2.0.0 (was 3.0.0-dev)"));
+  });
+}