refactor(AppRootUrl): simplify the code

Closes #4563

modules/angular2/src/core/compiler/anchor_based_app_root_url.ts

@@ -11,11 +11,8 @@ export class AnchorBasedAppRootUrl extends AppRootUrl {
   constructor() {
     super("");
     // compute the root url to pass to AppRootUrl
-    var rootUrl: string;
     var a = DOM.createElement('a');
     DOM.resolveAndSetHref(a, './', null);
-    rootUrl = DOM.getHref(a);
-
-    this.value = rootUrl;
+    this.value = DOM.getHref(a);
   }
 }

modules/angular2/src/core/compiler/app_root_url.ts

@@ -12,14 +12,5 @@ import {isBlank} from 'angular2/src/core/facade/lang';
  */
 @Injectable()
 export class AppRootUrl {
-  private _value: string;
-
-  constructor(value: string) { this._value = value; }
-
-  /**
-   * Returns the base URL of the currently running application.
-   */
-  get value() { return this._value; }
-
-  set value(value: string) { this._value = value; }
+  constructor(public value: string) {}
 }