Commits on Sep 6, 2019

1. Do not put mutable state on the prototype. …

   Using a mutable object on the addon's prototype causes issues when ember-m3 is consumed by an addon as a direct dependency.
   
   The specific setup is:
   
   * `ember-m3` depends on ember-compatibility-helpers
   * `ember-m3` has the following in its `index.js`:
   
   ```js
   module.exports = {
     name: require('./package').name,
   
     options: {
       m3: true,
       babel: {
         loose: true,
       },
     },
   };
   ```
   
   * addon B depends on `ember-m3` (**not** a dev dep)
   
   In this scenario, `ember-m3` will be instantiated twice. Once for addon B's dummy app and again for addon B's direct dependency. Since `ember-m3` had a _mutable object_ on its prototype (yes, you know whats coming now) ember-compatibility-helpers adds a `plugins` array to it and pushes into it.
   
   Now that ember-m3 has updated to Babel 7, pushing the same babel plugin into the plugins array twice triggers an error (which is super annoying to debug):
   
   ```
   Build Error (broccoli-persistent-filter:Babel > [Babel: ember-m3]) in ember-m3/factory.js
   
   Duplicate plugin/preset detected.
   If you'd like to use two separate instances of a plugin,
   they need separate names, e.g.
   
     plugins: [
       ['some-plugin', {}],
       ['some-plugin', {}, 'some unique name'],
     ]
   
   Stack Trace and Error Report: /var/folders/mt/yt2qjpl93ls98lrkrs81rzch000pzv/T/error.dump.246a0987138182f85c209aed25484574.log
   ```

   

   rwjblue committed Sep 6, 2019
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