groovy.sql.Sql throws ClassNotFoundException when called via GroovyShell that explicitly adds jdbc jar to classpath

The following script fails with a ClassNotFoundException. The script has no problem instantiating the com.mysql.jdbc.Driver class, but groovy.sql.Sql can't do the same. This may be related to GROOVY-3413.

import java.net.URL
import java.net.URLClassLoader

def cp = []
cp << new URL("file:mysql-connector-java-5.1.6-bin.jar")
def loader = new URLClassLoader(cp as URL[], this.class.classLoader)

def shell = new GroovyShell(loader)

def script = """
assert null != new com.mysql.jdbc.Driver()
"""
shell.evaluate(script)

script = """
import groovy.sql.Sql
assert null != Sql.newInstance("jdbc:mysql://localhost:3306/db", "user", "pass", "com.mysql.jdbc.Driver")
"""
shell.evaluate(script)