Make "--no-packages-dir" the default.

test/unknown_source_test.dart

Index: test/unknown_source_test.dart
diff --git a/test/unknown_source_test.dart b/test/unknown_source_test.dart
index 65bc476ebba69efdcf191a4a03fe6fbeb9676ebe..89c66f3ef2000a4003b3560a42f2562f2b9cac9f 100644
--- a/test/unknown_source_test.dart
+++ b/test/unknown_source_test.dart
@@ -60,11 +60,9 @@ main() {
       pubCommand(command);
 
       // Should upgrade to the new one.
-      d.dir(packagesPath, [
-        d.dir("foo", [
-          d.file("foo.dart", 'main() => "foo";')
-        ])
-      ]).validate();
+      d.appPackagesFile({
+        "foo": "../foo"
+      }).validate();
     });
   });
 }