_decideFunctionDeclarationType(node) {
    if (!this._isTopDepthInBody(node, this._ast.program.body)) return {type: null, node: null};

    return {type: 'Function', node: node};
  }
decide Doc type from function declaration node.
@param {ASTNode} node - target node that is function declaration node.
@returns {{type: string, node: ASTNode}} decided type.
@private