Syntax error in MySQL migration to drop constraint (patch supplied)

I have a migration that adds constraints correctly. When migrating down, however, I get a syntax error

  Error #1 - SQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'CONSTRAINT unique_username_idx' at line 1. Failing Query: "ALTER TABLE conUser DROP CONSTRAINT unique_username_idx"

Here is the down migration:

    public function down() {
        $this->dropConstraint('conUser', 'unique_username_idx');
    }

The SQL generated is "ALTER TABLE conUser DROP CONSTRAINT unique_username_idx". This post (http://forums.mysql.com/read.php?98,70887,70974#msg-70974) suggests that in MySQL the syntax should be "ALTER TABLE conUser DROP INDEX unique_username_idx" as constraints are basically indexes in MySQL (unlike MSSQL and Oracle). Doctrine's lib/Doctrine/Export/MySql.php appears to have a syntax error in the dropConstraint method. I attach a patch for this, but the only change is replacing "$name = 'CONSTRAINT '" with "$name = 'INDEX '" in dropConstraint. The migration then runs as I would expect.

Sorry if this has been fixed elsewhere, I did a search but couldn't find a similar ticket.