Convert ActionCable javascript to ES2015 modules with modern build environment

We've replaced the sprockets `//= require` directives with ES2015
imports. As a result, the ActionCable javascript can now be compiled
with rollup (like ActiveStorage already is).

- Rename action_cable/index.js.erb -> action_cable/index.js

- Add rake task to generate a javascript module of the ActionCable::INTERNAL ruby hash

  This will allow us to get rid of ERB from the actioncable javascript,
  since it is only used to interpolate ActionCable::INTERNAL.to_json.

- Import INTERNAL directly in ActionCable Connection module

  This is necessary to remove a load-order dependency conflict in the
  rollup-compiled build. Using ActionCable.INTERNAL would result in a
  runtime error:
  ```
  TypeError: Cannot read property 'INTERNAL' of undefined
  ```
  because ActionCable.INTERNAL is not set before the Connection module
  is executed.

  All other ActionCable.* references are executed inside of the body of a
  function, so there is no load-order dependency there.

- Add eslint and eslint-plugin-import devDependencies to actioncable

  These will be used to add a linting setup to actioncable like the one
  in activestorage.

- Add .eslintrc to actioncable

  This lint configuration was copied from activestorage

- Add lint script to actioncable

  This is the same as the lint script in activestorage

- Add babel-core, babel-plugin-external-helpers, and babel-preset-env devDependencies to actioncable

  These will be used to add ES2015 transpilation support to actioncable
  like we have in activestorage.

- Add .babelrc to actioncable

  This configuration was copied from activestorage

- Enable loose mode in ActionCable's babel config

  This generates a smaller bundle when compiled

- Add rollup devDependencies to actioncable

  These will be used to add a modern build pipeline to actioncable like
  the one in activestorage.

- Add rollup config to actioncable

  This is essentially the same as the rollup config from activestorage

- Add prebuild and build scripts to actioncable package

  These scripts were copied from activestorage

- Invoke code generation task as part of actioncable's prebuild script

  This will guarantee that the action_cable/internal.js module is
  available at build time (which is important, because two other modules
  now depend on it).

- Update actioncable package to reference the rollup-compiled files

  Now that we have a fully functional rollup pipeline in actioncable, we
  can use the compiled output in our npm package.

- Remove build section from ActionCable blade config

  Now that rollup is responsible for building ActionCable, we can remove
  that responsibility from Blade.

- Remove assets:compile and assets:verify tasks from ActionCable

  Now that we've added a compiled ActionCable bundle to version control,
  we don't need to compile and verify it at publish-time.

  (We're following the pattern set in ActiveStorage.)

- Include compiled ActionCable javascript bundle in published gem

  This is necessary to maintain support for depending on the ActionCable
  javascript through the Sprockets asset pipeline.

- Add compiled ActionCable bundle to version control

  This mirrors what we do in ActiveStorage, and allows ActionCable to
  continue to be consumed via the sprockets-based asset pipeline when
  using a git source instead of a published version of the gem.

1 files changed, 20 insertions, 3 deletions

diff --git a/actioncable/package.json b/actioncable/package.json
index d9df46043d..5d5105a6d5 100644
--- a/actioncable/package.json
+++ b/actioncable/package.json
@@ -2,9 +2,9 @@
   "name": "actioncable",
   "version": "6.0.0-alpha",
   "description": "WebSocket framework for Ruby on Rails.",
-  "main": "lib/assets/compiled/action_cable.js",
+  "main": "app/assets/javascripts/action_cable.js",
   "files": [
-    "lib/assets/compiled/*.js"
+    "app/assets/javascripts/*.js"
   ],
   "repository": {
     "type": "git",
@@ -20,5 +20,22 @@
   "bugs": {
     "url": "https://github.com/rails/rails/issues"
   },
-  "homepage": "http://rubyonrails.org/"
+  "homepage": "http://rubyonrails.org/",
+  "devDependencies": {
+    "babel-core": "^6.25.0",
+    "babel-plugin-external-helpers": "^6.22.0",
+    "babel-preset-env": "^1.6.0",
+    "eslint": "^4.3.0",
+    "eslint-plugin-import": "^2.7.0",
+    "rollup": "^0.58.2",
+    "rollup-plugin-babel": "^3.0.4",
+    "rollup-plugin-commonjs": "^9.1.0",
+    "rollup-plugin-node-resolve": "^3.3.0",
+    "rollup-plugin-uglify": "^3.0.0"
+  },
+  "scripts": {
+    "prebuild": "yarn lint && bundle exec rake assets:codegen",
+    "build": "rollup --config rollup.config.js",
+    "lint": "eslint app/javascript"
+  }
 }