anchor.diff:     * Throws [ArgumentError] if configuration with [name] does not exists.
      */
     Map get(name) {
  -    if (_configurations.containsKey(name)) throw new ArgumentError("Configuration does not exist");
  +    if (!_configurations.containsKey(name)) throw new ArgumentError("Configuration does not exist");
       var config = _configurations[name];
       if (config["parent"] == null) {
  -
  +      return config["configMap"].toMap();
  +    } else {
  +      return mergeMaps(get(config["parent"]), config["configMap"]);
       }
     }
   }
positive.code: 
  class Configuration {
  
    static final Map<String, Map > _configurations = {};
  
    /**
     * Register configuration under [name].
     */
    void add(String name, Map configuration, {String parent}) {
      _configurations[name] = {
        "parent": parent,
        "configMap": new ConfigMap(other:configuration)
  
negative.code: 
  class ComputedValue {
    final _computation;
    bool _evaluated = false;
    dynamic _value;
    ComputedValue(dynamic this._computation(ConfigMap config));
  
  