Grunt Uglify cannot access code within a closure - gruntjs

Question

Edit: this issue actually seems to be specific to uglifyjs itself. Running the code in an online tool like http://lisperator.net/uglifyjs/ gives the same results
I've got a grunt task that simply concatenates some files and then uses uglify to minify the resulting file.
file a:
console.log('this is file a');
file b:
(function () {
console.log('this is file b');
});
The concat works fine and I get concatenated file in my dist directory (testout.js) that looks like this:
(function () {
console.log('this is file b');
});
console.log('this is file a');
The the uglify task runs and produces testdist.min.js. Which looks like this:
console.log("this is file a");
None of the code from file b is included! As soon as I remove the closure from file b the second console.log is included in uglify's output.
Here's the entire grunt file.
module.exports = function (grunt) {
grunt.initConfig({
pkg: grunt.file.readJSON('package.json'),
concat: {
test: {
src: [
'src/filea.js',
'src/fileb.js'
],
dest: 'dist/testout.js'
},
},
uglify: {
thing: {
files: {
'dist/testdist.min.js' : ['dist/testout.js']
}
}
},
watch: {
js: {
files: ['src/*.js'],
tasks: ['concat','uglify']
}
}
});
grunt.loadNpmTasks('grunt-contrib-concat');
grunt.loadNpmTasks('grunt-contrib-uglify');
grunt.loadNpmTasks('grunt-contrib-watch');
grunt.loadNpmTasks('grunt-contrib-jshint');
grunt.registerTask('default', ['watch']);
}