anchor.diff:  Matcher isUnminifiedDart2JSOutput =
       contains("// The code supports the following hooks");
   
  -/// The directory containing the version of barback that should be used for this
  -/// test.
  -String _barbackDir;
  +/// A map from package names to paths from which those packages should be loaded
  +/// for [createLockFile].
  +///
  +/// This allows older versions of dependencies than those that exist in the repo
  +/// to be used when testing pub.
  +Map<String, String> _packageOverrides;
   
   /// A map from barback versions to the paths of directories in the repo
   /// containing them.
  
positive.code:   for (var version in validVersions) {
      group("with barback $version", () {
        setUp(() {
          _barbackDir = _barbackVersions[version];
        });
  
        callback();
  
negative.code: /// This includes the latest version of barback from pkg as well as all old
  /// versions of barback in third_party.
  final _barbackVersions = _findBarbackVersions();
  
  /// Some older barback versions require older versions of barback's dependencies
  /// than those that are in the repo.
  ///
  /// This is a map from barback version ranges to the dependencies for those
  /// barback versions. Each dependency version listed here should be included in
  /// third_party/pkg.
  final _barbackDeps = {
    new VersionConstraint.parse("<0.15.0"): {
      "source_maps": "0.9.4"
    }
  };
  
  /// Populates [_barbackVersions].
  Map<Version, String> _findBarbackVersions() {
  