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

test/downgrade/doesnt_change_git_dependencies_test.dart

Index: test/downgrade/doesnt_change_git_dependencies_test.dart
diff --git a/test/downgrade/doesnt_change_git_dependencies_test.dart b/test/downgrade/doesnt_change_git_dependencies_test.dart
index 0fa29443543294a9a7fc92e62cc651ebe3452394..c4ac3b961fc32bb451ab0504b5bcf3fc23c14530 100644
--- a/test/downgrade/doesnt_change_git_dependencies_test.dart
+++ b/test/downgrade/doesnt_change_git_dependencies_test.dart
@@ -18,7 +18,9 @@ main() {
       "foo": {"git": "../foo.git"}
     }).create();
 
-    pubGet();
+    // TODO(rnystrom): Remove "--packages-dir" and validate using the
+    // ".packages" file instead of looking in the "packages" directory.
+    pubGet(args: ["--packages-dir"]);
 
     d.dir(packagesPath, [
       d.dir('foo', [
@@ -31,7 +33,9 @@ main() {
       d.libPubspec('foo', '1.0.0')
     ]).commit();
 
-    pubDowngrade();
+    // TODO(rnystrom): Remove "--packages-dir" and validate using the
+    // ".packages" file instead of looking in the "packages" directory.
+    pubDowngrade(args: ["--packages-dir"]);
 
     d.dir(packagesPath, [
       d.dir('foo', [