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, 25 insertions, 28 deletions

diff --git a/actioncable/app/javascript/action_cable/subscription.js b/actioncable/app/javascript/action_cable/subscription.js
index 95b9ff6042..7de08f93b3 100644
--- a/actioncable/app/javascript/action_cable/subscription.js
+++ b/actioncable/app/javascript/action_cable/subscription.js
@@ -55,38 +55,35 @@
 //
 // The "AppearanceChannel" name is automatically mapped between the client-side subscription creation and the server-side Ruby class name.
 // The AppearanceChannel#appear/away public methods are exposed automatically to client-side invocation through the perform method.
-ActionCable.Subscription = (function() {
-  const extend = function(object, properties) {
-    if (properties != null) {
-      for (let key in properties) {
-        const value = properties[key]
-        object[key] = value
-      }
-    }
-    return object
-  }
 
-  class Subscription {
-    constructor(consumer, params = {}, mixin) {
-      this.consumer = consumer
-      this.identifier = JSON.stringify(params)
-      extend(this, mixin)
+const extend = function(object, properties) {
+  if (properties != null) {
+    for (let key in properties) {
+      const value = properties[key]
+      object[key] = value
     }
+  }
+  return object
+}
 
-    // Perform a channel action with the optional data passed as an attribute
-    perform(action, data = {}) {
-      data.action = action
-      return this.send(data)
-    }
+export default class Subscription {
+  constructor(consumer, params = {}, mixin) {
+    this.consumer = consumer
+    this.identifier = JSON.stringify(params)
+    extend(this, mixin)
+  }
 
-    send(data) {
-      return this.consumer.send({command: "message", identifier: this.identifier, data: JSON.stringify(data)})
-    }
+  // Perform a channel action with the optional data passed as an attribute
+  perform(action, data = {}) {
+    data.action = action
+    return this.send(data)
+  }
 
-    unsubscribe() {
-      return this.consumer.subscriptions.remove(this)
-    }
+  send(data) {
+    return this.consumer.send({command: "message", identifier: this.identifier, data: JSON.stringify(data)})
   }
 
-  return Subscription
-})()
+  unsubscribe() {
+    return this.consumer.subscriptions.remove(this)
+  }
+}