function parseForIn(node, init) {
    node.left = init;
    node.right = parseExpression();
    expect(_parenR);
    node.body = parseStatement();
    labels.pop();
    return finishNode(node, "ForInStatement");
  }
Parse a `for`/`in` loop.

@param {!node_t} node
@param {!node_t} init
@returns {!node_t}