function isCallNode(node) {
  // Example node: {
  //   type: 'ExpressionStatement',
  //   expression: { type: 'CallExpression', callee: {...}, arguments: [...] },
  // }
  return node.type == 'ExpressionStatement' &&
         node.expression.type == 'CallExpression';
}
@param {ASTNode} node A node from the abstract syntax tree.
@return {boolean} true if this is a call node.