Using API doesn't add HMR plugin entry

[rlamana]

Hot Module Replacement plugin entry doesn't seem to be added when using the API. The new tests on the PR #1664 use the dev server API via the test/helper.js and addEntries function in lib/utils/addEntries.js had to be manually called to make it work.

webpack Version: 4.29.0
webpack-dev-server Version: 3.1.14

• This is a bug
• This is a modification request

How can we reproduce the behavior?

Remove addEntries call in test/Client.test.js and the e2e test will fail because HMR is not loaded.

Expected behavior

When using the API and hot is enabled, HMR plugin entry should be also added.

[knagaitsev]

@evilebottnawi I messed with this issue briefly here: https://github.com/Loonride/webpack-dev-server/blob/hmr-entry-fix/lib/Server.js#L131. It would be nice if we had the ability to create the webpack compiler within Server.js. The problem I ran into was that users can use the compiler after they pass it in, so redefining it in Server.js does not work: https://github.com/webpack/webpack-dev-server/blob/master/test/Server.test.js#L25

[alexander-akait]

@Loonride yep, we should think how to better solve this without break other code, feel free to experiment and send a PR