Validates and adds a column to a table in schema.
Source
void addColumn(String tableName, SchemaColumn column, {String unencodedInitialValue}) {
var table = schema.tableForName(tableName);
if (table == null) {
throw new SchemaException("Table $tableName does not exist.");
}
table.addColumn(column);
if (store != null) {
commands.addAll(store.addColumn(table, column, unencodedInitialValue: unencodedInitialValue));
}
}