Migrate CommonJS to ES2015 module notation (imports and exports)

[jsnmoon]

Context

Porting CommonJS modules to ES2015 modules will allow us to take advantage of Webpack 2's tree shaking optimization.

Status

Completed

• Improve 5to6-codemod's import comment handling
• Write a codemod to generate named exports from default object import (awaiting publication)
• Improve CommonJS import codemod's comment handling
• Improve CommonJS import codemod's called import handling
• Write a codemod to hoist imports in blocks to the top
• Test running codemods on client/
• Enhance the named export generation codemod to transform functions to export functions
• Provide easy executables for Calypso developers to ES2015-ify their modules (Add codemods: CommonJS -> ES2015 module #12520)

In review

Backburner

• Integrate eslint-plugin-import into project (Check module import validity with eslint-plugin-import #12303)
• Fix Recast bug for the extra blank line between imports
• Create a streamlined way to execute codemods (previous attempt:run-codemods Add run-codemods script #13163)

Logs and Notes

Initial impressions

We have many occurrences of conditional imports and imports inside blocks. Neither of these are allowed in ES modules, so we'll need to migrate these eventually.

• Dynamic imports are possible, but probably not stable enough for us to jump on.
• Hoisting imports from the blocks to the top might be straightforward solution. I would need to figure out how much of these imports have side effects before going forward with this, though. Most imports inside a block are from controller.js files.

Progress - 03/01/2017

• Tested porting CommonJS imports via regex.
• Tested porting CommonJS imports/exports via 5to6-codemod.

Progress - 03/24/2017

• Contributed improvements to 5to6-codemod relating to both import and export transforms; currently awaiting merge & NPM publish for new version.
• PR in progress to add 5to6-codemod as executable scripts inside this project.
• PR in progress for enhancing our ESLint with eslint-plugin-import.

Progress - 04/04/2017

• Made various improvements to 5to6-codemod, including CommonJS import hoisting and improved reliability.
• All three codemods (import, export, and export name generation) run error-free for files inside client/.

Progress - 04/17/2017

• Refactored run-codemods into a separate PR.

[jsnmoon]

Update: named export generation codemod has been enhanced.

[jsnmoon]

Update: bin/run-codemods has been created.

[jsnmoon]

Update: Added progress update for 04/17/2017 covering run-codemods PR. react-codemods PR and upstream PR for reactjs/react-codemod.

[jsnmoon]

Update: Removed all mentions of react-codemods. Please refer to #13321 if you'd like to follow along the React migration project!

[jsnmoon]

This issue is in a good place to close for now. If anyone has any interest in picking up the backburner items, please ping!