Istanbul coverage for babel based nodejs repo

.babelrc

{
  "presets": [
    "es2015",
    "stage-3"
  ],
  "env": {
    "test": {
      "plugins": [
        [
          "istanbul",
          {
            "exclude": [
              "**/*.spec.js"
            ]
          }
        ]
      ]
    }
  }
}

dumb.js

class Library {
  doThing(source) {
    if (source) {
      return source;
    }
    return 'result';
  }
}

export const library = () =>
  new Library();

dumb.spec.js

import 'babel-polyfill';
import { expect } from 'chai';
import { library } from './dumb';

describe('Library', () => {
  let lib;

  beforeEach(() => {
    lib = library();
  });

  it('should do thing', () => {
    let ret = lib.doThing();
    expect(ret).to.exist;
  });

  it('should echo source', () => {
    let source = 'Hi';
    let ret = lib.doThing(source);
    expect(ret).to.equal(source);
  });
});

package.json

{
  "name": "test",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test:single": "mocha --compilers js:babel-core/register --recursive **/*.spec.js",
    "test": "NODE_ENV=test nyc --reporter=lcov --reporter=text npm run test:single",
    "precoverage": "rm -Rf coverage"
  },
  "nyc": {
    "sourceMap": false,
    "instrument": false
  },
  "keywords": [],
  "author": "Ha Pham <mahpahh@gmail.com> (http://mahpahh.com)",
  "license": "ISC",
  "devDependencies": {
    "babel-cli": "^6.16.0",
    "babel-core": "^6.17.0",
    "babel-plugin-istanbul": "^2.0.3",
    "babel-polyfill": "^6.16.0",
    "babel-preset-es2015": "^6.16.0",
    "babel-preset-stage-3": "^6.17.0",
    "chai": "^3.5.0",
    "istanbul": "^0.4.5",
    "mocha": "^3.1.2",
    "nyc": "^8.3.1"
  }
}