1) Deleted package com.google.javascript.jscomp.ant to avoid dependency on ant.jar. 
2) jarjar-ed it:
    <target name="jarjarClosureCompiler">
        <taskdef name="jarjar" classname="com.tonicsystems.jarjar.JarJarTask" classpath="${basedir}/dependencies/jarjar.jar"/>

        <delete file="${output}/closure-compiler.jar" failonerror="false"/>
        <jarjar jarfile="${output}/closure-compiler.jar">
            <zipgroupfileset dir="${basedir}" includes="closure-compiler.jar" />

            <rule pattern="com.google.common.**" result="closurecompiler.internal.com.google.common.@1"/>
            <rule pattern="com.google.protobuf.**" result="closurecompiler.internal.com.google.protobuf.@1"/>
            <rule pattern="org.mozilla.classfile.**" result="closurecompiler.internal.org.mozilla.classfile.@1"/>
        </jarjar>
    </target>
