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(); }