_unwrapExportDeclaration(node) {
    // e.g. `export A from './A.js'` has not declaration
    if (!node.declaration) return null;

    const exportedASTNode = node.declaration;
    if (!exportedASTNode.leadingComments) exportedASTNode.leadingComments = [];
    exportedASTNode.leadingComments.push(...node.leadingComments || []);

    if (!exportedASTNode.trailingComments) exportedASTNode.trailingComments = [];
    exportedASTNode.trailingComments.push(...node.trailingComments || []);

    return exportedASTNode;
  }
unwrap exported node.
@param {ASTNode} node - target node that is export declaration node.
@returns {ASTNode|null} unwrapped child node of exported node.
@private