ApplicationInstance should create container

Based on feedback from @stefanpenner, we moved responsibility for
creating the default container from the `Application` to the
`ApplicationInstance`. Instead, the instance points back at the
application’s registry, using it as the basis for a new container.

This commit also adds initial documentation to the application instance.

packages/ember-application/lib/system/application-instance.js

@@ -1,16 +1,43 @@
 /**
 @module ember
 @submodule ember-application
+@private
 */
 
 import EmberObject from "ember-runtime/system/object";
 import run from "ember-metal/run_loop";
 
+/**
+  The `ApplicationInstance` encapsulates all of the stateful aspects of a
+  running `Application`.
+
+  At a high-level, we break application boot into two distinct phases:
+
+  * Definition time, where all of the classes, templates, and other
+    dependencies are loaded (typically in the browser).
+  * Run time, where we begin executing the application once everything
+    has loaded.
+
+  Definition time can be expensive and only needs to happen once since it is
+  an idempotent operation. For example, between test runs and FastBoot
+  requests, the application stays the same. It is only the state that we want
+  to reset.
+
+  That state is what the `ApplicationInstance` manages: it is responsible for
+  creating the container that contains all application state, and disposing of
+  it once the particular test run or FastBoot request has finished.
+*/
+
 export default EmberObject.extend({
-  container: null,
+  registry: null,
   customEvents: null,
   rootElement: null,
 
+  init: function() {
+    this._super();
+    this.container = this.registry.container();
+  },
+
   startRouting: function(isModuleBasedResolver) {
     var router = this.container.lookup('router:main');
     if (!router) { return; }
@@ -33,6 +60,7 @@ export default EmberObject.extend({
   },
 
   willDestroy: function() {
+    this._super.apply(this, arguments);
     run(this.container, 'destroy');
   }
 });

packages/ember-application/lib/system/application.js

@@ -267,6 +267,7 @@ var Application = Namespace.extend(DeferredMixin, {
     // Create subclass of Ember.Router for this Application instance.
     // This is to ensure that someone reopening `App.Router` does not
     // tamper with the default `Ember.Router`.
+    // 2.0TODO: Can we move this into a globals-mode-only library?
     this.Router = Router.extend();
     this.buildRegistry();
 
@@ -300,14 +301,15 @@ var Application = Namespace.extend(DeferredMixin, {
     @return {Ember.Container} the configured container
   */
   buildInstance: function() {
-    var container = this.__container__ = this.__registry__.container();
-
     var instance = this.__instance__ = ApplicationInstance.create({
       customEvents: get(this, 'customEvents'),
       rootElement: get(this, 'rootElement'),
-      container: container
+      registry: this.__registry__
     });
 
+    // TODO2.0: Legacy support for App.__container__
+    this.__container__ = instance.container;
+
     return instance;
   },