karma-webpack-warnings-plugin.js

var webpackConfig = {
    plugins: [
        // Plugin to show any webpack warnings and prevent tests from running
        function () {
            this.plugin("done", function (stats) {
                if (stats.compilation.warnings.length) {
                    // Log each of the warnings
                    stats.compilation.warnings.forEach(function (warning) {
                        console.log(warning.message || warning);
                    });

                    // Pretend no assets were generated. This prevents the tests
                    // from running making it clear that there were warnings.
                    stats.stats = [{
                        toJson: function () {
                            return this;
                        },
                        assets: []
                    }];
                }
            });
        }
    ]
}

here's my solution for webpack v4 - https://gist.github.com/jonesmac/9ef456153c714db56be0ec24b61c6fbb