fix(router): load route config from async instructions

Previously, async routes generated from links would not load the configs of
their resolved components, which led to broken links in the children of the
async instruction's component.

This commit fixes the bookkeeping in the Router to correctly load the configs.

Fixes internal b/23791558

Closes #4146

modules/angular2/test/router/route_registry_spec.ts

@@ -235,7 +235,7 @@ export function main() {
       registry.config(RootHostCmp,
                       new Route({path: '/first/...', component: DummyParentCmp, as: 'first'}));
       expect(() => { registry.generate(['first'], RootHostCmp); })
-          .toThrowError('Link "["first"]" does not resolve to a terminal instruction.');
+          .toThrowError('Link "["first"]" does not resolve to a terminal or async instruction.');
     });
 
     it('should match matrix params on child components and query params on the root component',