Deprecate Application Controller Router Properties

text/0421-deprecate-application-controller-props.md

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+- Start Date: 2018-19-12
+- RFC PR: https://github.com/emberjs/rfcs/pull/421
+- Ember Issue: (leave this empty)
+
+# Deprecate Application Controller Router Properties
+
+## Summary 
+
+This RFC proposes the deprecation of `ApplicationController#currentPath` and `ApplicationController#currentRouteName`.
+
+## Motivation
+
+These APIs are no longer needed as the `RouterService` now has `RouterService#currentPath` and `RouterService#currentRouteName`.
+These fields are only ever present on the application controller which is a weird special casing that we would like to remove.
+Additionally, it's likely that there are very few if any consumers of this API as it is not documented.
+
+## Transition Path
+
+If you are reliant on `ApplicationController#currentPath` and `ApplicationController#currentRouteName` you can get the same functionality from the `RouterService` to migrate, inject the `RouterService` and read the `currentRouteName` or `currentPath` off of it.
+
+Before:
+
+```js
+// app/controllers/application.js
+import Controller from '@ember/controller';
+import fetch from 'fetch';
+
+export default Controller.extend({
+  store: service('store'),
+
+  actions: {
+    sendPayload() {
+      fetch('/endpoint', {
+        method: 'POST',
+        body: JSON.stringify({
+          route: this.currentRouteName
+        })
+      });
+    }
+  }
+})
+```
+
+After:
+
+```js
+// app/controllers/application.js
+import Controller from '@ember/controller';
+import fetch from 'fetch';
+
+export default Controller.extend({
+  store: service('store'),
+  router: service('router'),
+
+  actions: {
+    sendPayload() {
+      fetch('/endpoint', {
+        method: 'POST',
+        body: JSON.stringify({
+          route: this.router.currentRouteName
+        })
+      });
+    }
+  }
+})
+```
+
+## How We Teach This
+
+There is likely very few consumers of this functionality and migration path is covered by existing documentation.
+
+## Drawbacks
+
+This may introduce churn that we are not aware of.
+
+## Alternatives
+
+No real alternative other than keep setting the properties.
+
+## Unresolved questions
+
+Optional, but suggested for first drafts. What parts of the design are still
+TBD?