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

    return {type: 'Function', node: node};
  }
decide Doc type from function expression node.
babylon 6.11.2 judges`export default async function foo(){}` to be `FunctionExpression`.
I expect `FunctionDeclaration`. this behavior may be bug of babylon.
for now, workaround for it with this method.
@param {ASTNode} node - target node that is function expression node.
@returns {{type: string, node: ASTNode}} decided type.
@private
@todo inspect with newer babylon.