Support "pub downgrade".

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

+// Copyright (c) 2014, 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("downgrades a dependency to the lowest matching version", () {
+    servePackages([
+      packageMap("foo", "1.0.0"),
+      packageMap("foo", "2.0.0-dev"),
+      packageMap("foo", "2.0.0"),
+      packageMap("foo", "2.1.0"),
+    ]);
+
+    d.appDir({"foo": ">=2.0.0 <3.0.0"}).create();
+
+    pubGet();
+
+    d.packagesDir({
+      "foo": "2.1.0",
+    }).validate();
+
+    pubDowngrade();
+
+    // 1.0.0 shouldn't be chosen because it's outside the version constraint and
+    // 2.0.0-dev shouldn't be chosen because release versions should be
+    // preferred.
+    d.packagesDir({
+      "foo": "2.0.0",
+    }).validate();
+
+    // The lockfile should be updated.
+    d.dir(appPath, [
+      d.matcherFile("pubspec.lock", contains("2.0.0"))
+    ]).validate();
+  });
+}