We have run into issues in the past where the actioncable compiled
javascript bundle got out of sync with the source code. For example, in
30a0c7e0 only the compiled bundle was
modified. This meant that anyone who ran `yarn build` in the actioncable
directory would then see a dirty git status indicating changes to the
compiled bundle, despite not having made any changes to the actioncable
javascript source code. We fixed that particular inconsistency in
a4c27588. However, the same problem
could reoccur.

To address this, I've added a new test to enforce that actioncable's
compiled javascript bundle is in sync with the source code. When the
compiled bundle is in sync with the source code, the test will pass:

    $ bundle exec ruby -Itest test/javascript_package_test.rb
    Run options: --seed 19308

    # Running:

    yarn run v1.12.3
    $ yarn lint && bundle exec rake assets:codegen
    $ eslint app/javascript
    $ rollup --config rollup.config.js

    app/javascript/action_cable/index.js → app/assets/javascripts/action_cable.js...
    created app/assets/javascripts/action_cable.js in 762ms
    ✨  Done in 6.35s.
    .

    Finished in 7.130345s, 0.1402 runs/s, 0.1402 assertions/s.
    1 runs, 1 assertions, 0 failures, 0 errors, 0 skips

However, if the two are not in sync, the test will fail. For example, if
you were to apply the following patch (which only updates the source
code):

```
diff --git a/actioncable/app/javascript/action_cable/adapters.js b/actioncable/app/javascript/action_cable/adapters.js
index 4de8131438..d38d9a6a0b 100644
--- a/actioncable/app/javascript/action_cable/adapters.js
+++ b/actioncable/app/javascript/action_cable/adapters.js
@@ -1,4 +1,5 @@
 export default {
+  foo: self.foo,
   logger: self.console,
   WebSocket: self.WebSocket
 }
```

the test would then fail like this:

    $ bundle exec ruby -Itest test/javascript_package_test.rb
    Run options: --seed 26377

    # Running:

    yarn run v1.12.3
    $ yarn lint && bundle exec rake assets:codegen
    $ eslint app/javascript
    $ rollup --config rollup.config.js

    app/javascript/action_cable/index.js → app/assets/javascripts/action_cable.js...
    created app/assets/javascripts/action_cable.js in 776ms
    ✨  Done in 5.55s.
    F

    Failure:
    JavascriptPackageTest#test_compiled_code_is_in_sync_with_source_code [test/javascript_package_test.rb:16]:
    --- expected
    +++ actual
    @@ -3,6 +3,7 @@
     })(this, function(exports) {
       \"use strict\";
       var adapters = {
    +    foo: self.foo,
         logger: self.console,
         WebSocket: self.WebSocket
       };

    rails test test/javascript_package_test.rb:9

    Finished in 5.837403s, 0.1713 runs/s, 0.1713 assertions/s.
    1 runs, 1 assertions, 1 failures, 0 errors, 0 skips

Thus, the actioncable test suite will now prevent "the compiled bundle
is out of sync" issues going forward.