refactor(ES6): upgrade Compilation to ES6

[ahmedelgabri]

What kind of change does this PR introduce?

ES5 => ES6 refactor

Did you add tests for your changes?

all tests are passing

Summary

Refactoring to ES2015/6

Does this PR introduce a breaking change?

No

[TheLarkInn]

Impressive work. Well done.

[TheLarkInn]

Thank you so much. This class was not an easy refactor. 🙇 👏 🍾

[ahmedelgabri]

Thank you so much. This class was not an easy refactor. 🙇 👏 🍾

It's just too long 😄

[ahomatthew]

@ahmedelgabri I think you unintentionally introduced a bug where if you are missing a file that is imported _this is prematurely being set to null before the error callback happens.

Here -> https://github.com/webpack/webpack/blob/master/lib/Compilation.js#L222 _this is set to null by the time it fires, and therefore triggers quite the red herring. Line where it's being set to null -> https://github.com/webpack/webpack/blob/master/lib/Compilation.js#L363

For background, i'm running a yarn build with create-react-app's react-scripts build. I'm doing an import on a relative path to a json file in my project.

What should happen:

sally@sunshine:/path/to/code$ yarn build
yarn run v1.1.0
$ yarn run build-css && react-scripts build
Rendering Complete, saving .css file...
Wrote CSS to /path/to/code/src/index.css
Wrote 1 CSS files to /path/to/code/src/
Creating an optimized production build...
Failed to compile.

Module not found: Error: Can't resolve './config.json' in '/path/to/code/src'

error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

What's actually happening:

sally@sunshine:/path/to/code$ yarn build
yarn run v1.1.0
$ yarn run build-css && react-scripts build
Rendering Complete, saving .css file...
Wrote CSS to /path/to/code/src/index.css
Wrote 1 CSS files to /path/to/code/src/
Creating an optimized production build...
/path/to/code/node_modules/webpack/lib/Compilation.js:219
				_this.errors.push(err);
				     ^

TypeError: Cannot read property 'errors' of null
    at errorAndCallback (/path/to/code/node_modules/webpack/lib/Compilation.js:219:10)
    at errorOrWarningAndCallback (/path/to/code/node_modules/webpack/lib/Compilation.js:251:14)
...

[ahmedelgabri]

@ahomatthew I'm not sure if it was me or is it related to something else because when I look at the file before my changes It's exactly the same after I converted it:

_this = null

webpack/lib/Compilation.js

Line 330 in 7dae8fb

_this = null;

_this.errors.push(err);

webpack/lib/Compilation.js

Line 198 in 7dae8fb

_this.errors.push(err);

[ahomatthew]

Ah got it, I'll file a bug.