assetsConvertor.js

var burrito = require('burrito'),
    mime = require('mime'),
    fs = require('fs'),
    src;

src = burrito('var image = hoge.fuga.game.assets["chara1.gif"]', function (node) {
    var p = node.parent(), n, fileName, fileData, mimeType;
    if (!p) {
        return;
    }
    n = p.value[0];
    if (node.name === 'string' && p.name === 'sub' && n.length === 3 && n[0] === 'dot' && n[2] === 'assets') {
        fileName = node.value[0];
        fileData = fs.readFileSync(fileName).toString('base64');
        mimeType = mime.lookup(fileName);
        node.wrap('"data:' + mimeType + ';base64,' + fileData + '"');
    }
});

console.log(src);