function parseFor(node, init) {
    node.init = init;
    expect(_semi);
    node.test = tokType === _semi ? null : parseExpression();
    expect(_semi);
    node.update = tokType === _parenR ? null : parseExpression();
    expect(_parenR);
    node.body = parseStatement();
    labels.pop();
    return finishNode(node, "ForStatement");
  }
Parse a regular `for` loop. The disambiguation code in `parseStatement`
will already have parsed the init statement or expression.

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