warningsFilter only effective when `sourceMap` option is truthy

package.json

{
  "devDependencies": {
    "webpack": "^2.6.0"
  }
}

test.js

"use strict"

var foo = "FOO";

webpack.config.js

var webpack = require('webpack');

module.exports = {
  entry: {
    "test": "./test.js",
  },

  output: {
    filename: "[name].bundle.js",
  },

  plugins: [
    new webpack.optimize.UglifyJsPlugin({
      compress: true,
      warningsFilter: function() { return false; },
      sourceMap: false, // Warnings disappear when sourceMap is set to true
    }),
  ],
};