app.js

console.log('app');

Demo.spec.js

import Vue from 'vue';
import Demo from '../src/views/Demo.vue';

describe('Demo.vue', () => {
  let vm = null;

  before(() => {
    const Ctor = Vue.extend(Demo);
    vm = new Ctor().$mount();
  });

  it('methods: foo()', () => {
    expect(vm.foo()).to.equal(42);
  });
});

Demo.vue

<template>
  <div>
    Hello World!
  </div>
</template>

<script>
export default {
  name: 'demo',
  methods: {
    foo() {
      return 42;
    },
  },
};
</script>

karma.conf.js

module.exports = (config) => {
  config.set({
    singleRun: true,
    frameworks: ['mocha', 'chai'],
    browsers: ['Chrome'],
    files: [
      'test/test.js',
    ],
    preprocessors: {
      'test/test.js': ['webpack', 'sourcemap'],
    },
    reporters: ['spec', 'coverage-istanbul'],
    coverageIstanbulReporter: {
      reports: ['html'],
      dir: './coverage',
      fixWebpackSourcePaths: true,
    },
    webpackMiddleware: { noInfo: true },
    webpack: {
      devtool: '#inline-source-map',
      module: {
        loaders: [
          {
            test: /\.js$/,
            loader: 'istanbul-instrumenter-loader',
            exclude: /node_modules/,
            include: /src/,
            enforce: 'post',
            options: {
              esModules: true,
            },
          },
          {
            test: /\.vue$/,
            loader: 'vue-loader',
            options: {
              preLoaders: {
                js: 'istanbul-instrumenter-loader?esModules=true',
              },
            },
          },
        ],
      },
    },
  });
};

package.json

{
  "name": "repro-vue-loader-895-issue",
  "version": "0.0.1",
  "scripts": {
    "test": "karma start"
  },
  "dependencies": {
    "vue": "^2.3.4"
  },
  "devDependencies": {
    "chai": "^4.1.0",
    "css-loader": "^0.28.4",
    "istanbul-instrumenter-loader": "^3.0.0",
    "karma": "^1.7.0",
    "karma-chai": "^0.1.0",
    "karma-chrome-launcher": "^2.2.0",
    "karma-coverage-istanbul-reporter": "^1.3.0",
    "karma-mocha": "^1.3.0",
    "karma-sourcemap-loader": "^0.3.7",
    "karma-spec-reporter": "0.0.31",
    "karma-webpack": "^2.0.4",
    "mocha": "^3.4.2",
    "vue-loader": "13.0.3",
    "vue-template-compiler": "^2.3.4",
    "webpack": "^3.2.0"
  }
}

test.js

const testsContext = require.context('./', true, /\.spec\.js$/);
testsContext.keys().forEach(testsContext);

const srcContext = require.context('../src', true);
srcContext.keys().forEach(srcContext);