Support "pub downgrade".

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

-// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// 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 '../../descriptor.dart' as d;
-import '../../test_pub.dart';
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
 
 main() {
   initConfig();
-  integration("upgrades one locked pub server package's dependencies if it's "
+  integration("downgrades one locked hosted package's dependencies if it's "
       "necessary", () {
     servePackages([
-      packageMap("foo", "1.0.0", {"foo_dep": "any"}),
-      packageMap("foo_dep", "1.0.0")
+      packageMap("foo", "2.0.0", {"foo_dep": "any"}),
+      packageMap("foo_dep", "2.0.0")
     ]);
 
     d.appDir({"foo": "any"}).create();
 
     pubGet();
 
     d.packagesDir({
+      "foo": "2.0.0",
+      "foo_dep": "2.0.0"
+    }).validate();
+
+    servePackages([
+      packageMap("foo", "1.0.0", {"foo_dep": "<2.0.0"}),
+      packageMap("foo_dep", "1.0.0")
+    ]);
+
+    pubDowngrade(args: ['foo']);
+
+    d.packagesDir({
       "foo": "1.0.0",
       "foo_dep": "1.0.0"
     }).validate();
-
-    servePackages([
-      packageMap("foo", "2.0.0", {"foo_dep": ">1.0.0"}),
-      packageMap("foo_dep", "2.0.0")
-    ]);
-
-    pubUpgrade(args: ['foo']);
-
-    d.packagesDir({
-      "foo": "2.0.0",
-      "foo_dep": "2.0.0"
-    }).validate();
   });
 }