fix(router): provide a top-level route segment for injection

modules/@angular/router/src/directives/router_link.ts

@@ -59,7 +59,7 @@ export class RouterLink implements OnDestroy {
   @HostBinding() href: string;
   @HostBinding('class.router-link-active') isActive: boolean = false;
 
-  constructor(@Optional() private _routeSegment: RouteSegment, private _router: Router) {
+  constructor(private _routeSegment: RouteSegment, private _router: Router) {
     // because auxiliary links take existing primary and auxiliary routes into account,
     // we need to update the link whenever params or other routes change.
     this._subscription =

modules/@angular/router/src/recognize.ts

@@ -8,7 +8,6 @@ import {ComponentResolver} from '@angular/core';
 import {DEFAULT_OUTLET_NAME} from './constants';
 import {reflector} from '@angular/core';
 
-// TODO: vsavkin: recognize should take the old tree and merge it
 export function recognize(componentResolver: ComponentResolver, rootComponent: Type,
                           url: UrlTree): Promise<RouteTree> {
   let matched = new _MatchResult(rootComponent, [url.root], {}, rootNode(url).children, []);

modules/@angular/router/src/router_providers_common.ts

@@ -1,6 +1,7 @@
 import {OpaqueToken, ComponentResolver} from '@angular/core';
 import {LocationStrategy, PathLocationStrategy, Location} from '@angular/common';
 import {Router, RouterOutletMap} from './router';
+import {RouteSegment} from './segments';
 import {RouterUrlSerializer, DefaultRouterUrlSerializer} from './router_url_serializer';
 import {ApplicationRef} from '@angular/core';
 import {BaseException} from '@angular/core';
@@ -18,6 +19,7 @@ export const ROUTER_PROVIDERS_COMMON: any[] = /*@ts2dart_const*/[
     deps: /*@ts2dart_const*/
         [ApplicationRef, ComponentResolver, RouterUrlSerializer, RouterOutletMap, Location],
   },
+  /*@ts2dart_Provider*/ {provide: RouteSegment, useFactory: (r) => r.routeTree.root, deps: [Router]}
 ];
 
 function routerFactory(app: ApplicationRef, componentResolver: ComponentResolver,