function getArgumentFromCallNode(idx, node) {
  // Example node: {
  //   type: 'ExpressionStatement',
  //   expression: { type: 'CallExpression', callee: {...}, arguments: [...] },
  // }
  assert(isCallNode(node));
  return node.expression.arguments[idx].value;
}
@param {number} idx An argument index from the call node.
@param {ASTNode} node A node from the abstract syntax tree.
@return {string} The argument value as a string.