Fix: ES5 client

[knagaitsev]

• This is a bugfix
• This is a feature
• This is a code refactor
• This is a test update
• This is a docs update
• This is a metadata update

For Bugs and Features; did you add new tests?

Yes

Motivation / Use-Case

We need to make the client work with ES5 for webpack@4. This PR does some case-specific patches, rather than trying to fully solve the problem, since webpack@5 always has ES6 syntax in its bundles, unless the user uses babel-loader.

The webpack/lib/logging/runtime and strip-ansi modules use ES6, so they are placed in the transpiled-modules directory and have files that act as a proxy entry to them. Then, these entry files are turned into a bundle with babel-loader, so that the result is a module that works with ES5 and behaves the same way.

Explanation for the ES5 check test: Since webpack takes all the JavaScript and puts it into a string in an eval function, a bundle that will not work with ES5 can still pass a basic ES5 check, since all this bad ES6 syntax is inside of a string. The best course of action for a test I think is to locate these strings in a bundle that eval is being run on, then check that they parse with ES5.

#1286 Actually was not fixed on v4 for the reason above, but should be fixed by this PR.

If you look at the test run below only after the first 2 commits, you can see that the test does not pass because there are still modules being used that don't work with ES5: https://github.com/webpack/webpack-dev-server/pull/2658/files/d01ac6855c0bc20f23054c9ca8832eea2539588d

Breaking Changes

None, client API is the same

Additional Info

[codecov]

Codecov Report

Merging #2658 into v4 will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##               v4    #2658      +/-   ##
==========================================
+ Coverage   92.85%   92.86%   +0.01%     
==========================================
  Files          37       39       +2     
  Lines        1301     1304       +3     
  Branches      348      349       +1     
==========================================
+ Hits         1208     1211       +3     
  Misses         89       89              
  Partials        4        4              

Impacted Files	Coverage Δ
client-src/default/index.js	92.47% <100.00%> (ø)
client-src/default/utils/log.js	100.00% <100.00%> (ø)
client-src/transpiled-modules/log.js	100.00% <100.00%> (ø)
client-src/transpiled-modules/strip-ansi.js	100.00% <100.00%> (ø)

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[knagaitsev]

Looks like test instability again on Windows.

[hiroppy]

still LGTM 👍

[alexander-akait]

I am afraid, some modules can use new syntax (ES 6) and we can't catch it, for example in patch version

[knagaitsev]

@evilebottnawi If that happens when we are updating a dependency, it will be caught in CI here: https://github.com/webpack/webpack-dev-server/pull/2658/files#diff-24488ac612ba9666465a7a596040ac73R43

[knagaitsev]

But if we want full safety, we should do this: #2652. I don't like how that PR ended up because it has so many breaking changes in the API and seems a little risky

[hiroppy]

Sound good to me. CI is enough for me.

[knagaitsev]

Ah, issue with lodash is making lint fail: https://www.npmjs.com/advisories/1523

[hiroppy]

@Loonride could you rebase this? This PR will be merged after merging #2675

[knagaitsev]

/cc @evilebottnawi @hiroppy