How do I integrate PostCSS into Broccoli angular-cli Build? - broccolijs

Question

It seems like this build script is running but the CSS that is output is not minified or auto prefixed. I am attempting to compile SASS, then run the output through Post CSS with Broccoli and the angular-cli. I figure maybe some ember-cli folks could help as well. What am I doing wrong?
The build outputs this in terminal:
Slowest Trees | Total
----------------------------------------------+---------------------
BroccoliTypeScriptCompiler | 1274ms
vendor | 502ms
PostcssFilter | 465ms
but the CSS is the same as if it were output from SASS, not Post CSS.
Here is my angular-cli-build.js:
'use strict';
/* global require, module */
const Angular2App = require('angular-cli/lib/broccoli/angular2-app');
const compileSass = require('broccoli-sass');
const compileCSS = require('broccoli-postcss');
const cssnext = require('postcss-cssnext');
const cssnano = require('cssnano');
const mergeTrees = require('broccoli-merge-trees');
const Funnel = require('broccoli-funnel');
const _ = require('lodash');
const glob = require('glob');
var options = {
plugins: [
{
module: cssnext,
options: {
browsers: ['> 1%'],
warnForDuplicates: false
}
},
{
module: cssnano,
options: {
safe: true,
sourcemap: true
}
}
]
};
module.exports = function(defaults) {
let sourceDir = 'src';
let appTree = new Angular2App(defaults, {
sourceDir: sourceDir,
sassCompiler: {
includePaths: [
'src/style'
]
},
vendorNpmFiles: [
'systemjs/dist/system-polyfills.js',
'systemjs/dist/system.src.js',
'zone.js/dist/*.js',
'es6-shim/es6-shim.js',
'reflect-metadata/*.js',
'reflect-metadata/*.js.map',
'rxjs/**/*.js',
'#angular/**/*.js',
'rxjs/**/*.js.map',
'#angular/**/*.js.map',
'd3/d3.js',
'three/build/three.js',
'three/examples/js/postprocessing/*.js',
'three/examples/js/shaders/*.js'
]
});
let sass = mergeTrees(_.map(glob.sync('src/**/*.scss'), function(sassFile) {
sassFile = sassFile.replace('src/', '');
return compileSass(['src'], sassFile, sassFile.replace(/.scss$/, '.css'));
}));
let css = compileCSS(sass, options);
return mergeTrees([sass, css, appTree], { overwrite: true });
};

Answer 1

It was the order of operations, my trees were not overwriting appTree!
This works!
return mergeTrees([appTree, sass, css], { overwrite: true });