Support "pub downgrade".

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

Index: sdk/lib/_internal/pub/test/downgrade/unlock_dependers_test.dart
diff --git a/sdk/lib/_internal/pub/test/upgrade/hosted/unlock_dependers_test.dart b/sdk/lib/_internal/pub/test/downgrade/unlock_dependers_test.dart
similarity index 67%
copy from sdk/lib/_internal/pub/test/upgrade/hosted/unlock_dependers_test.dart
copy to sdk/lib/_internal/pub/test/downgrade/unlock_dependers_test.dart
index b217321b9b74c5c129a70d42614b40938f6ddc0d..93a376c0416817c2c228b05419f203101128273e 100644
--- a/sdk/lib/_internal/pub/test/upgrade/hosted/unlock_dependers_test.dart
+++ b/sdk/lib/_internal/pub/test/downgrade/unlock_dependers_test.dart
@@ -4,16 +4,16 @@
 
 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 a locked package's dependers in order to get it to max "
-      "version", () {
+  integration("downgrades a locked package's dependers in order to get it to "
+      "min version", () {
     servePackages([
-      packageMap("foo", "1.0.0", {"bar": "<2.0.0"}),
-      packageMap("bar", "1.0.0")
+      packageMap("foo", "2.0.0", {"bar": ">1.0.0"}),
+      packageMap("bar", "2.0.0")
     ]);
 
     d.appDir({"foo": "any", "bar": "any"}).create();
@@ -21,20 +21,20 @@ main() {
     pubGet();
 
     d.packagesDir({
-      "foo": "1.0.0",
-      "bar": "1.0.0"
+      "foo": "2.0.0",
+      "bar": "2.0.0"
     }).validate();
 
     servePackages([
-      packageMap("foo", "2.0.0", {"bar": "<3.0.0"}),
-      packageMap("bar", "2.0.0")
+      packageMap("foo", "1.0.0", {"bar": "any"}),
+      packageMap("bar", "1.0.0")
     ]);
 
-    pubUpgrade(args: ['bar']);
+    pubDowngrade(args: ['bar']);
 
     d.packagesDir({
-      "foo": "2.0.0",
-      "bar": "2.0.0"
+      "foo": "1.0.0",
+      "bar": "1.0.0"
     }).validate();
   });
 }