Fixes #3775

Upgrade Babel 6 to Babel 7 (major)

* Initial implementation of next export without exportPathMap

* Shorter message

* Set up flow

* Create pages manifest

* Use pagesManifest for next export

* Fix tests

* Document defaultPathMap

* Replacing the path is no longer needed

* Use posix normalize for consistent behaviour

* Remove second instance of examples

* Add comment about what pages-manifest does

* Make windows path a route

Here we don't use the full capabilities of babel-preset-env.
But it's recommended to use it without browser configs as
a substitute for babel-preset-latest.
That's why we did this.

* Run tests serially.

* Make test result verbose.

* Don't wait until closing the browser.

* Add some debug logs.

* Add bailing support.

* Get the browser with a timeout.

* Add some comments.

* Remove istanbul babel tranformation.
Jest already do it and it's breaking our coveralls hit.

* Move the test babel setup to the main babelrc file.

* Add es2015 preset.

* Use jest-cli instead of gulp plugin.

* Use jest-cli instead of gulp plugin.

* Move fixtures into the examples dir.

* Move test code of example app to the basic example.

* Add isolated tests for server/resolve

* Allow tests to use cheerio.

* Use portfinder to get a unique port.

* Move back integration tests into the example dir.

* Introduce next-test-utils.

* Remove gulp-jest

* Add coveralls support.

* Use transpiled version of code in dist.
This is to make sure same file gets covered
by both unit/isolated tests and integration tests.

* Add support for source maps.

* Use code from dist always.

* Use nyc to stop instrument.

* Add integration test suite for production usage.

* Use jest-cli.

* Add support for running e2e tests.

* Check gzipPath with fs.stat before serving
Otherwise, serve package might throw issues other than ENOENT

* Install chromedriver with npm install.

* Install chrome on travis-ci.

* Add --forceExit to Jest.

* Run tests only on Node v6.
That's because selenium-webdriver only supports
Node 6 LTS.

* Use chromedriver NPM module to install chromedriver.

* Use wd as the webdriver client.

* Run chromedriver before tests.

* Run travis for both node 4 and 6

* Remove unwanted npm install script.

* Move some common text utilities to next-test-utils

* Add lint checks and testing in npm prepublish hook.

* Use npm on travis-ci.
We are having some caching issues with yarn and chromedriver.

* Make tests work on windows.\n But chromedriver doesn't work.

* Clean up dependencies.

* Run chromedriver in background without any tools.

* Fix a typo in the code.

* Use ES6 features used in node4 inside the gulpfile.

* Add some comments.

* Add support for running in windows.

* Stop chromedriver properly on windows.

* Fix typos.