anchor.diff:          key: (prop) => prop.name,
           value: (PropertyDefinition prop) => prop.type.create(initialValue: propertyInits[prop.name])
       );
  -  
  +
     @override
  -  _PropertyInstance operator [](String key) => _entityProperties[key];
  +  _PropertyInstance operator [](String key) {
  +    if (!containsKey(key))
  +      throw new NoSuchPropertyError(kind, key);
  +    return _entityProperties[key];
  +  }
   
     @override
     bool containsKey(String key) => _entityProperties.containsKey(key);
  
positive.code:    * or `null` if no entity corresponding with the given key exists.
     */
    final T entity;
    
    bool get isKeyOnlyResult => resultType == KEY_ONLY;
    
    bool get isPresent => resultType == ENTITY_PRESENT;
    
    final resultType;
    
    EntityResult._(this.resultType, this.key, this.entity);
    
    factory EntityResult._fromSchemaEntityResult(
        Datastore datastore,
        schema.EntityResult entityResult, 
        schema.EntityResult_ResultType resultType) {
      if (resultType == schema.EntityResult_ResultType.KEY_ONLY) {
        var key = new Key._fromSchemaKey(entityResult.entity.key);
  
negative.code:   final Key key;
    KindDefinition get kind => datastore.kindByName(key.kind);
    final PropertyMap _properties;
  
    /**
     * Create a new [Entity] against the given [datastore]
     * with the given [key] and, optionally, initial values
  