Make servePackages's contents argument saner.

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/downgrade/unlock_dependers_test.dart b/sdk/lib/_internal/pub/test/downgrade/unlock_dependers_test.dart
index 93a376c0416817c2c228b05419f203101128273e..c329bdb28a9b156ffb49e1000905e2dd7c21bb9c 100644
--- a/sdk/lib/_internal/pub/test/downgrade/unlock_dependers_test.dart
+++ b/sdk/lib/_internal/pub/test/downgrade/unlock_dependers_test.dart
@@ -11,10 +11,10 @@ main() {
   initConfig();
   integration("downgrades a locked package's dependers in order to get it to "
       "min version", () {
-    servePackages([
-      packageMap("foo", "2.0.0", {"bar": ">1.0.0"}),
-      packageMap("bar", "2.0.0")
-    ]);
+    servePackages((builder) {
+      builder.serve("foo", "2.0.0", deps: {"bar": ">1.0.0"});
+      builder.serve("bar", "2.0.0");
+    });
 
     d.appDir({"foo": "any", "bar": "any"}).create();
 
@@ -25,10 +25,10 @@ main() {
       "bar": "2.0.0"
     }).validate();
 
-    servePackages([
-      packageMap("foo", "1.0.0", {"bar": "any"}),
-      packageMap("bar", "1.0.0")
-    ]);
+    servePackages((builder) {
+      builder.serve("foo", "1.0.0", deps: {"bar": "any"});
+      builder.serve("bar", "1.0.0");
+    });
 
     pubDowngrade(args: ['bar']);