Grunt babel multiple files and preserve source mapping

I am trying, using grunt and babel, to transpile all js6 files in a folder and end up with a concatenated single file (js5) with a working sourcemap to the original es6 files. However the sourcemapping does not work. My babel, concat settings below:

 "babel": {
        options: {
            sourceMap : true
        },
        dist: {
            files:[
                {
                    expand: true,
                    cwd: 'wwwroot/js/src',
                    src: ['*.js'],
                    dest: 'tmp/js'
                }]
        }
    },

    concat: {
        options: {
            sourceMap: true
        },
        js: {
            src: [
              'tmp/js/*.js',
            ],
            dest: 'wwwroot/js/app.js'
        }
    }

Versions:
"grunt": "0.4.5",
"grunt-bower-task": "0.4.0",
"grunt-babel": "5.0.1",
"grunt-contrib-concat" : "0.5.1"

I am ending up with firstly a folder with a lot of js files and src maps(tmp directory). But concatinating them into one file messes up with source mapping completely.

Ideas? Also, can I somehow skip the making of temporary files and sort of just pipe the result into concat?

Reversing the order of task will make this much easier.First run the concat task on the JS files. After that run babel task on the single file created by concat task previously with the following options

options: {
                sourceMap: true,
                inputSourceMap: grunt.file.readJSON('script.js.map')
            },

Here the script.js.map file is the name of the source map file generated by concat task. As inputSourceMap option excepts a source map object , we pass it in using the grunt.file API's readJSON method

The full Grunt file configuration would be:

concat: {
        options: {
            sourceMap: true
        },
        js: {
            src: ['Modules/**/js/*.js'],
            dest: 'script.js'
        }
    },
    babel: {
        dist: {
            options: {
                sourceMap: true,
                inputSourceMap: grunt.file.readJSON('script.js.map')
            },
            src: [
                'script.js',
            ],
            dest: 'app.js'
        }
    }

Example project: https://github.com/pra85/Grunt-Concat-Babel-Example