c9-core/node_modules/c9/setup_paths.js

/**
 * Allow code that lives on the server to resolve paths like
 * require("plugins/c9.language/language"),
 * just like on the client.
 */
var modules = require("module");
var oldResolve = modules._resolveFilename;
var extraPaths = [
    __dirname + "/../../"
];
modules._resolveFilename = function(request, parent) {
    if (!parent) parent = { paths: [], filename: "" };
    // Ensure client extensions can be loaded
    request = request.replace(/^lib\//, "node_modules/");
    // Add the extra paths
    extraPaths.forEach(function(p) {
        if (parent.paths.indexOf(p) === -1)
            parent.paths.push(p);
    });
    return oldResolve(request, parent);
};