Get included template element

@inner
@param {Object} tplFile the included template file
@return {?Object}
function getIncludeTemplateElement(tplFile) {
    let ast = tplFile.ast;
    if (!ast) {
        ast = tplFile.ast = parseDom(tplFile.content.toString());
    }

    let children = ast.children;
    for (let i = 0, len = children.length; i < len; i++) {
        let node = children[i];
        if (node.type === 'tag' && node.name === 'template') {
            return node;
        }
    }
}