pushLine = function(line) {
    body.push(indentation + line + (includeSemicolonAndReturn ? ';' : ''));
  }
Produces code that when run extracts an index key from a row.
Because this function is used both for single/cross column indices, and for
non-nullable/nullable indices it needs to be customized to produce the
correct output in both situations.
@param {!lf.schema.Table} table
@param {!lf.schema.Column} column
@param {string} indentation The prefix to use for each line.
@param {boolean} includeSemicolonAndReturn Whether the output should include
    semicolons and 'return' keywords.
@return {string}
@private