Schema.fromMigrations constructor
Create a schema from a set of migrations. If version is not provided,
the highest migration version will be used
Implementation
factory Schema.fromMigrations(Set<Migration> migrations, [int? version]) {
  assert((version == null) || (version > -1));
  version = version ?? MigrationManager.latestMigrationVersion(migrations);
  final commands = expandMigrations(migrations);
  final tables = commands.fold(<SchemaTable>{}, _commandToSchema);
  return Schema(
    version,
    tables: tables,
  );
}