_decideMethodDefinitionType(node) {
    const classNode = this._findUp(node, ['ClassDeclaration', 'ClassExpression']);
    if (this._processedClassNodes.includes(classNode)) {
      return {type: 'Method', node: node};
    } else {
      logger.w('this method is not in class', node);
      return {type: null, node: null};
    }
  }
decide Doc type from method definition node.
@param {ASTNode} node - target node that is method definition node.
@returns {{type: ?string, node: ?ASTNode}} decided type.
@private