Support "pub downgrade".

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

Index: sdk/lib/_internal/pub/test/downgrade/doesnt_change_git_dependencies_test.dart
diff --git a/sdk/lib/_internal/pub/test/get/git/doesnt_fetch_if_nothing_changes_test.dart b/sdk/lib/_internal/pub/test/downgrade/doesnt_change_git_dependencies_test.dart
similarity index 57%
copy from sdk/lib/_internal/pub/test/get/git/doesnt_fetch_if_nothing_changes_test.dart
copy to sdk/lib/_internal/pub/test/downgrade/doesnt_change_git_dependencies_test.dart
index 54cd98c62e249139c31d33629878c382d0868699..b661c7dc4c6bef28939fcccf9c4f2a19fbcfa270 100644
--- a/sdk/lib/_internal/pub/test/get/git/doesnt_fetch_if_nothing_changes_test.dart
+++ b/sdk/lib/_internal/pub/test/downgrade/doesnt_change_git_dependencies_test.dart
@@ -2,28 +2,25 @@
 // 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:path/path.dart' as p;
+import 'package:path/path.dart' as path;
 import 'package:scheduled_test/scheduled_test.dart';
 
-import '../../../lib/src/io.dart';
-import '../../descriptor.dart' as d;
-import '../../test_pub.dart';
+import '../../lib/src/io.dart';
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
 
 main() {
   initConfig();
-  integration("doesn't re-fetch a repository if nothing changes", () {
+  integration("doesn't change git dependencies", () {
     ensureGit();
 
-    var repo = d.git('foo.git', [
+    d.git('foo.git', [
       d.libDir('foo'),
       d.libPubspec('foo', '1.0.0')
-    ]);
-    repo.create();
+    ]).create();
 
     d.appDir({
-      "foo": {"git": {"url": "../foo.git"}}
+      "foo": {"git": "../foo.git"}
     }).create();
 
     pubGet();
@@ -34,11 +31,12 @@ main() {
       ])
     ]).validate();
 
-    // Delete the repo. This will cause "pub get" to fail if it tries to
-    // re-fetch.
-    schedule(() => deleteEntry(p.join(sandboxDir, 'foo.git')));
+    d.git('foo.git', [
+      d.libDir('foo', 'foo 2'),
+      d.libPubspec('foo', '1.0.0')
+    ]).commit();
 
-    pubGet();
+    pubDowngrade();
 
     d.dir(packagesPath, [
       d.dir('foo', [