anchor.diff:  /// 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() {
  
positive.code: 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 barback versions to the paths of directories in the repo
  /// containing them.
  
negative.code:   for (var version in validVersions) {
      group("with barback $version", () {
        setUp(() {
          _packageOverrides = {};
          _packageOverrides['barback'] = _barbackVersions[version];
          _barbackDeps.forEach((constraint, deps) {
            if (!constraint.allows(version)) return;
            deps.forEach((packageName, version) {
              _packageOverrides[packageName] = path.join(
                  repoRoot, 'third_party', 'pkg', '$packageName-$version');
            });
          });
  
          currentSchedule.onComplete.schedule(() {
            _packageOverrides = null;
          });
        });
  
        callback();
  