Plugin loading fails when using syncImport

Loading a plugin fails when less is using the syncImport option. The File manager seems to return the result instead of a promise in this case. It can be easily reproduced with the following code:

Example:

// pi.js
module.exports = {
    install: function(less, pluginManager, functions) {
        functions.add('pi', function() {
            return Math.PI;
        });
    }
};

// index.js
var less = require('less');

less.render('@plugin "./pi.js"; @var pi()', { syncImport: true }, function (error, output) {
    console.log(error, output);
});

Error:

TypeError: fileManager.loadFile(...).then is not a function
    at /home/ydelange/less-bug/node_modules/less/lib/less-node/plugin-loader.js:34:72
    at new Promise (<anonymous>)
    at PluginLoader.loadPlugin (/home/ydelange/less-bug/node_modules/less/lib/less-node/plugin-loader.js:33:12)
    at ImportManager.push (/home/ydelange/less-bug/node_modules/less/lib/less/import-manager.js:149:36)
    at ImportVisitor.processImportNode (/home/ydelange/less-bug/node_modules/less/lib/less/visitors/import-visitor.js:88:28)
    at ImportVisitor.visitImport (/home/ydelange/less-bug/node_modules/less/lib/less/visitors/import-visitor.js:50:22)
    at Visitor.visit (/home/ydelange/less-bug/node_modules/less/lib/less/visitors/visitor.js:76:32)
    at Visitor.visitArray (/home/ydelange/less-bug/node_modules/less/lib/less/visitors/visitor.js:102:22)
    at Ruleset.accept (/home/ydelange/less-bug/node_modules/less/lib/less/tree/ruleset.js:40:30)
    at Visitor.visit (/home/ydelange/less-bug/node_modules/less/lib/less/visitors/visitor.js:83:18)

Code that is causing it:
https://github.com/less/less.js/blob/master/lib/less-node/file-manager.js#L42

Start with the syncImport reproduction in index.js and inspect the linked code in lib/less-node/file-manager.js alongside the failing call in lib/less-node/plugin-loader.js. Run the example with the pi.js plugin and confirm that loading no longer raises the .then TypeError and the render callback receives its result.