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

test/upgrade/git/upgrade_to_nonexistent_pubspec_test.dart

Index: test/upgrade/git/upgrade_to_nonexistent_pubspec_test.dart
diff --git a/test/upgrade/git/upgrade_to_nonexistent_pubspec_test.dart b/test/upgrade/git/upgrade_to_nonexistent_pubspec_test.dart
index 19918618bfb1a92305a2e02e9cb06ecb78bbdfe5..b794ef05b58a69488f6c8b55d49d74a86cec7a4c 100644
--- a/test/upgrade/git/upgrade_to_nonexistent_pubspec_test.dart
+++ b/test/upgrade/git/upgrade_to_nonexistent_pubspec_test.dart
@@ -17,7 +17,7 @@ main() {
 
     d.appDir({"foo": {"git": "../foo.git"}}).create();
 
-    pubGet();
+    pubGet(args: ["--packages-dir"]);
 
     d.dir(packagesPath, [
       d.dir('foo', [
@@ -28,8 +28,10 @@ main() {
     repo.runGit(['rm', 'pubspec.yaml']);
     repo.runGit(['commit', '-m', 'delete']);
 
-    pubUpgrade(error: new RegExp(r'Could not find a file named "pubspec.yaml" '
-        r'in [^\n]*\.'));
+    pubUpgrade(
+        args: ["--packages-dir"],
+        error: new RegExp(r'Could not find a file named "pubspec.yaml" '
+            r'in [^\n]*\.'));
 
     d.dir(packagesPath, [
       d.dir('foo', [