function assertColumnsPopulated(columns, relation) {
  relation.entries.forEach(
      function(entry, index) {
        columns.forEach(function(column) {
          // Checking that all requested columns are populated.
          assertTrue(entry.getField(column) != null);
        });
      });
}
Asserts that all requested columns are populated in the given relation's
entries.
@param {!Array<!lf.schema.Column>} columns
@param {!lf.proc.Relation} relation The relation to be checked.