ERROR [preprocess]: Can not load "webpack"! WebpackOptionValidationError

### Plugins

- Karma v1.3.0
- Karma-webpack v2.0.2
- Webpack v2.2.1

### Karma.config.js

var argv = require('yargs').argv;

var path = require('path');


module.exports = function (config) {

    config.set({

        browsers: ['Chrome'],

        singleRun: !argv.watch,

        frameworks: ['chai', 'mocha'],

        reporters: ['mocha', 'coverage'],

        mochaReporter: {

            output: 'autowatch'

        },

        coverageReporter: {

            reporters: [

                { type: 'html', dir: './coverage' },

                { type: 'text-summary' }

            ]

        },

        files: [

            './node_modules/babel-polyfill/dist/polyfill.js',

            {

                pattern: `./test_bundler.js`,

                watched: false,

                served: true,

                included: true

            }

        ],

        preprocessors: {

            ['./test_bundler.js']: ['webpack'],

            // ['./test_bundler.js']: ['webpack', 'sourcemap'] - sourcemaps are cool but really slow.

            // refer to sourcemaps built by "$ npm run build:dev" instead

        },

        port: 9876,

        webpack: {

            devtool: 'inline-source-map',

            resolve: {

                root: path.resolve(__dirname, './src'),

                extensions: ['', '.js', '.jsx', '.json'],

                alias: {

                    sinon: require.resolve('sinon/pkg/sinon')

                }

            },

            module: {

                noParse: [

                    /node_modules\/sinon\//

                ],

                preLoaders: [{

                    test: /\.js?$/,

                    exclude: /(test|node_modules|bower_components)\//,

                    loader: 'isparta-instrumenter-loader'

                }],

                loaders: [

                    {

                        test: /\.jsx?$/,

                        exclude: /node_modules/,

                        loader: 'babel'

                    },

                    {

                        test: /\.json$/,

                        loader: 'json'

                    },

                    {

                        test: /\.woff2?$|\.ttf$|\.eot$|\.svg$/,

                        loader: 'file'

                    },

                    {

                        test: /\.scss$/,

                        loader: 'style!css!sass'

                    },

                    {

                        test: /\.css$/,

                        loader: 'style!css'

                    }

                ]

            },

            externals: {

                'jsdom': 'window',

                'react/addons': true,

                'react/lib/ExecutionEnvironment': true,

                'react/lib/ReactContext': 'window'

            },

        },

        webpackMiddleware: {

            noInfo: true,

            stats: {

                chunks: false

            }

        },

        plugins: [

            'karma-mocha',

            'karma-chai',

            'karma-webpack',

            'karma-chrome-launcher',

            'karma-mocha-reporter',

            'karma-coverage'

            //'karma-sourcemap-loader'

        ]

    });

};

### Actual behaviour


Start with the Karma.config.js configuration and the referenced test_bundler.js entry point, then reproduce the preprocessing failure using the listed Karma, karma-webpack, and webpack versions. Compare the reported WebpackOptionValidationError with the webpack configuration shown; done means the test bundle loads without the preprocessing error.

Written by the indexing model from the issue text.