chore(typings): mark underscore-prefix members @internal

I still haven't made a tslint check for this, so new occurrences leaked in.
See https://github.com/angular/angular/issues/4645

Closes #4986

modules/angular2/src/core/application_ref.ts

@@ -215,6 +215,7 @@ export abstract class PlatformRef {
 export class PlatformRef_ extends PlatformRef {
   /** @internal */
   _applications: ApplicationRef[] = [];
+  /** @internal */
   _disposeListeners: Function[] = [];
 
   constructor(private _injector: Injector, private _dispose: () => void) { super(); }

modules/angular2/src/core/zone/ng_zone.ts

@@ -185,6 +185,7 @@ export class NgZone {
    */
   get onTurnStart(): /* Subject */ any { return this._onTurnStartEvents; }
 
+  /** @internal */
   _notifyOnTurnStart(parentRun): void {
     parentRun.call(this._innerZone, () => { this._onTurnStartEvents.next(null); });
   }
@@ -213,6 +214,7 @@ export class NgZone {
    */
   get onTurnDone() { return this._onTurnDoneEvents; }
 
+  /** @internal */
   _notifyOnTurnDone(parentRun): void {
     parentRun.call(this._innerZone, () => { this._onTurnDoneEvents.next(null); });
   }
@@ -251,6 +253,7 @@ export class NgZone {
    */
   get onEventDone() { return this._onEventDoneEvents; }
 
+  /** @internal */
   _notifyOnEventDone(): void {
     this.runOutsideAngular(() => { this._onEventDoneEvents.next(null); });
   }

modules/angular2/src/router/router.ts

@@ -478,6 +478,7 @@ export class Router {
 export class RootRouter extends Router {
   /** @internal */
   _location: Location;
+  /** @internal */
   _locationSub: Object;
 
   constructor(registry: RouteRegistry, location: Location, primaryComponent: Type) {