chore(typings): mark underscore methods @internal.

This allows TypeScript to produce an API surface which matches the Dart semantics.
I found these with:
gulp build.js.dev && find dist/js/dev/es5/angular2/src -name "*.d.ts" -exec grep -H -n '^ *_' {} \;

Closes #4638

modules/angular2/src/router/async_route_handler.ts

@@ -3,6 +3,7 @@ import {Promise, PromiseWrapper} from 'angular2/src/core/facade/async';
 import {isPresent, Type} from 'angular2/src/core/facade/lang';
 
 export class AsyncRouteHandler implements RouteHandler {
+  /** @internal */
   _resolvedComponent: Promise<any> = null;
   componentType: Type;