[BUGFIX] {{link-to}} before application boot

Previously, {{link-to}} would work even if the application had not been
booted completely. This behavior is relied on by test harnesses like
ember-qunit, since an “integration test” of a component may include a
template that uses {{link-to}}. Many people have discovered that they
would like to test a component that has a link without booting the
entire app.

> You wanted a banana but what you got was a gorilla holding the banana
> and the entire jungle. –Joe Armstrong

During the Glimmer work, we refactored the interface between the
LinkComponent and the router into a service. Unfortunately, this
caused a regression where rendering the link before the router was
completely initialized would raise an exception.

This commit adds a very stupid guard to ensure that we have enough
router state to determine if a link is active or not. This whole path
could be significantly cleaned up, and in particular, we should move
towards using a stubbed routing service in tests that encapsulates all
of these concerns.

Conflicts:
	packages/ember/tests/helpers/link_to_test.js

packages/ember-routing-views/lib/views/link.js

@@ -259,6 +259,8 @@ var LinkComponent = EmberComponent.extend({
   */
   active: computed('attrs.params', '_routing.currentState', function computeLinkComponentActive() {
     var currentState = get(this, '_routing.currentState');
+    if (!currentState) { return false; }
+
     return computeActive(this, currentState);
   }),
 

packages/ember/tests/helpers/link_to_test.js

@@ -1,4 +1,5 @@
 import "ember";
+import ComponentLookup from "ember-views/component_lookup";
 
 import { objectControllerDeprecation } from "ember-runtime/controllers/object_controller";
 import EmberHandlebars from "ember-htmlbars/compat";
@@ -93,6 +94,30 @@ QUnit.module("The {{link-to}} helper", {
   teardown: sharedTeardown
 });
 
+// This test is designed to simulate the context of an ember-qunit/ember-test-helpers component integration test,
+// so the container is available but it does not boot the entire app
+QUnit.test('Using {{link-to}} does not cause an exception if it is rendered before the router has started routing', function(assert) {
+  Router.map(function() {
+    this.route('about');
+  });
+
+  registry.register('component-lookup:main', ComponentLookup);
+
+  let component = Ember.Component.extend({
+    layout: compile('{{#link-to "about"}}Go to About{{/link-to}}'),
+    container: container
+  }).create();
+
+  let router = container.lookup('router:main');
+  router.setupRouter();
+
+  Ember.run(function() {
+    component.appendTo('#qunit-fixture');
+  });
+
+  assert.strictEqual(component.$('a').length, 1, 'the link is rendered');
+});
+
 QUnit.test("The {{link-to}} helper moves into the named route", function() {
   Router.map(function(match) {
     this.route("about");