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

test/descriptor.dart

Index: test/descriptor.dart
diff --git a/test/descriptor.dart b/test/descriptor.dart
index d4855be06af4abc9a50c2d340bfc5a4423b00504..b55844c51c1d1adc8914fe8fd6d3a27d9c24340d 100644
--- a/test/descriptor.dart
+++ b/test/descriptor.dart
@@ -203,3 +203,11 @@ DirectoryDescriptor appDir([Map dependencies]) =>
 /// path to a path dependency, relative to the application directory.
 Descriptor packagesFile([Map<String, String> dependencies]) =>
   new PackagesFileDescriptor(dependencies);
+
+/// Describes a `.packages` file in the application directory, including the
+/// implicit entry for the app itself.
+Descriptor appPackagesFile(Map<String, String> dependencies) {
+  var copied = new Map<String, String>.from(dependencies);
+  copied["myapp"] = ".";
+  return dir(appPath, [packagesFile(copied)]);
+}