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

test/upgrade/git/upgrade_to_incompatible_pubspec_test.dart

Index: test/upgrade/git/upgrade_to_incompatible_pubspec_test.dart
diff --git a/test/upgrade/git/upgrade_to_incompatible_pubspec_test.dart b/test/upgrade/git/upgrade_to_incompatible_pubspec_test.dart
index 670a535d1ede8a1d9a9dbdb34f24273e114321b4..846961a7862a23a08680c00b74dc90ddf088f1d3 100644
--- a/test/upgrade/git/upgrade_to_incompatible_pubspec_test.dart
+++ b/test/upgrade/git/upgrade_to_incompatible_pubspec_test.dart
@@ -19,7 +19,7 @@ main() {
 
     d.appDir({"foo": {"git": "../foo.git"}}).create();
 
-    pubGet();
+    pubGet(args: ["--packages-dir"]);
 
     d.dir(packagesPath, [
       d.dir('foo', [
@@ -32,8 +32,10 @@ main() {
       d.libPubspec('zoo', '1.0.0')
     ]).commit();
 
-    pubUpgrade(error: contains('"name" field doesn\'t match expected name '
-        '"foo".'), exitCode: exit_codes.DATA);
+    pubUpgrade(
+        args: ['--packages-dir'],
+        error: contains('"name" field doesn\'t match expected name "foo".'),
+        exitCode: exit_codes.DATA);
 
     d.dir(packagesPath, [
       d.dir('foo', [