ERROR from UglifyJs Unexpected token keyword, expected punc - reactjs

Question

I'm getting the following error when trying to run npm install on my project: ERROR in bundle.js from UglifyJs
Unexpected token keyword «function», expected punc «,»
I'm aware it's because I'm trying to use an ES6 feature, but I think something is wrong with my setup which is why it isn't getting compiled to ES5 by Babel.
I've looked at a number of problems on SO regarding the issue, and I've incorporated some of those answers, but none has solved the problem.
Here is my package.json:
{
"name": "my-project",
"version": "0.0.1",
"engines": {
"node": "8.9.1"
},
"private": true,
"scripts": {
"dev": "webpack --progress --colors --watch",
"postinstall": "NODE_ENV=prod webpack -p"
},
"dependencies": {
"babel-core": "^6.7.4",
"babel-loader": "^7.1.2",
"babel-plugin-transform-react-jsx": "^6.1.18",
"babel-preset-es2015": "^6.6.0",
"babel-preset-react": "^6.5.0",
"body-parser": "^1.18.2",
"express": "^4.16.2",
"material-ui": "^0.20.0",
"react": "^16.2.0",
"react-dom": "^16.2.0",
"react-player": "^1.1.2",
"react-render": "^1.2.1",
"webpack": "^3.11.0",
"yargs": "^11.0.0"
},
"babel": {
"plugins": [
"transform-react-jsx"
],
"presets": [
"es2015",
"react"
]
},
"devDependencies": {
"uglifyjs-webpack-plugin": "^1.2.0"
}
}
and my webpack.config.babel.js
var UglifyJSPlugin = require('uglifyjs-webpack-plugin');
module.exports = {
context: __dirname + "/static",
entry: {
client: ['./App.js']
},
output: {
filename: 'bundle.js',
path: __dirname + "/static/build"
},
resolve: {
extensions: ['*', '.js']
},
plugins: [
new UglifyJSPlugin({
include: /\.min\.js$/
})
],
module: {
loaders: [
{
test: /(\.js$)/,
loader: 'babel-loader',
exclude: /node_modules/,
query: {
presets: [
'react',
['es2015', {'modules': false}]
]
}
}
]
}
};