code: function isDirective(node) {
  			return node.type === "ExpressionStatement" &&
  				node.expression.type === "Literal" &&
  				typeof node.expression.value === "string";
  		}
docs: Checks whether or not a given node is a directive.
  The directive is a `ExpressionStatement` which has only a string literal.
  @param {ASTNode} node - A node to check.
  @returns {boolean} Whether or not the node is a directive.
  @private