Backport get-owner API so plugins can use it safely

app/assets/javascripts/discourse-common/lib/deprecated.js.es6

@@ -0,0 +1,3 @@
+export default function deprecated(msg) {
+  console.warn(`DEPRECATION: ${msg}`);
+}

app/assets/javascripts/discourse-common/lib/get-owner.js.es6

@@ -0,0 +1,32 @@
+import deprecated from 'discourse-common/lib/deprecated';
+
+export function getOwner(obj) {
+  if (Ember.getOwner) {
+    return Ember.getOwner(obj) || Discourse.__container__;
+  }
+
+  return obj.container;
+}
+
+// `this.container` is deprecated, but we can still build a container-like
+// object for components to use
+export function getRegister(obj) {
+  const owner = getOwner(obj);
+  const register = {
+    lookup: (...args) => owner.lookup(...args),
+    lookupFactory: (...args) => {
+      return owner.lookupFactory ? owner.lookupFactory(...args) : owner._lookupFactory(...args);
+    },
+
+    deprecateContainer(target) {
+      Object.defineProperty(target, 'container', {
+        get() {
+          deprecated("Use `this.register` or `getOwner` instead of `this.container`");
+          return register;
+        }
+      });
+    }
+  };
+
+  return register;
+}