fix: es6 syntax in client

[alexander-akait]

• 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?

no, it is critical fix, test will be in future

Motivation / Use-Case

fixes #1981

Breaking Changes

No

Additional Info

/cc @hiroppy @Loonride I am temporary block merge any PRs before we refactor tests/tests strcuture/structure inside lib (for any branches)

Please don't touch next branch before we do this in master

[alexander-akait]

Test structure (approximately):

- test
  - e2e
  - cli
  - client
    - clients
      - SockJSClient.test.js 
    - overlay.test.js 
  - server
    - servers
      - SockJSServer.test.js
    - utils
      - tryParseInt.test.js

Test naming:

1. class tests have same name as class -> MyClass.js -> MyClass.test.js
2. pure file like defaultPort.js -> defaultPort.test.js
3. options for server should be placed in server directory and have name compress-option.test.js
4. options for client should be placed in client directory and have name sockPath-options.test.js

Next we should improve test performance, many tests are very long and written in effective way.

Some of tests files should be merge in other, some of separated based on above structure.

I am WIP on this after merge this PR

[alexander-akait]

Need rewrite tests

[codecov]

Codecov Report

Merging #1982 into master will decrease coverage by 0.19%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #1982     +/-   ##
=========================================
- Coverage   92.47%   92.27%   -0.2%     
=========================================
  Files          28       28             
  Lines        1076     1075      -1     
  Branches      322      322             
=========================================
- Hits          995      992      -3     
- Misses         77       79      +2     
  Partials        4        4

Impacted Files	Coverage Δ
client-src/clients/BaseClient.js	33.33% <ø> (ø)
client-src/clients/SockJSClient.js	86.66% <ø> (ø)
lib/utils/updateCompiler.js	100% <ø> (ø)	⬆️

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[alexander-akait]

Merge without review because it is critical, feel free to feedback

[hiroppy]

I would like to separate utils tests. Utils shouldn't have to test through the server. It is e2e.

[alexander-akait]

@hiroppy yes, utils should have own unit tests and in Server.test.js we should do integration test (example - check when we run server configuration has HotModuleReplacmentPlugin and etc)

[alexander-akait]

@hiroppy #1983