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:       if (dependencies.containsKey(package)) return;
  
        var packagePath;
        if (package == 'barback' && _packageOverrides == null) {
          throw new StateError("createLockFile() can only create a lock file "
              "with a barback dependency within a withBarbackVersions() "
              "block.");
        }
  
        if (_packageOverrides.containsKey(package)) {
          packagePath = _packageOverrides[package];
        } else {
          packagePath = path.join(pkgPath, package);
        }
  