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;
 

modules/angular2/src/router/location.ts

@@ -80,7 +80,9 @@ export const APP_BASE_HREF: OpaqueToken = CONST_EXPR(new OpaqueToken('appBaseHre
  */
 @Injectable()
 export class Location {
+  /** @internal */
   _subject: EventEmitter = new EventEmitter();
+  /** @internal */
   _baseHref: string;
 
   constructor(public platformStrategy: LocationStrategy,

modules/angular2/src/router/route_recognizer.ts

@@ -99,6 +99,7 @@ export class RouteRecognizer {
     return solutions;
   }
 
+  /** @internal */
   _redirect(urlParse: Url): Url {
     for (var i = 0; i < this.redirects.length; i += 1) {
       let redirector = this.redirects[i];

modules/angular2/src/router/router.ts

@@ -204,6 +204,7 @@ export class Router {
     });
   }
 
+  /** @internal */
   _navigate(instruction: Instruction, _skipLocationChange: boolean): Promise<any> {
     return this._settleInstruction(instruction)
         .then((_) => this._canReuse(instruction))
@@ -230,6 +231,7 @@ export class Router {
   // guaranteed that the `componentType`s for the terminal async routes have been loaded by the time
   // we begin navigation. The method below simply traverses instructions and resolves any components
   // for which `componentType` is not present
+  /** @internal */
   _settleInstruction(instruction: Instruction): Promise<any> {
     var unsettledInstructions: Array<Promise<any>> = [];
     if (isBlank(instruction.component.componentType)) {
@@ -257,6 +259,7 @@ export class Router {
   /*
    * Recursively set reuse flags
    */
+  /** @internal */
   _canReuse(instruction: Instruction): Promise<any> {
     if (isBlank(this._outlet)) {
       return _resolveToFalse;
@@ -335,8 +338,10 @@ export class Router {
   }
 
 
+  /** @internal */
   _startNavigating(): void { this.navigating = true; }
 
+  /** @internal */
   _finishNavigating(): void { this.navigating = false; }
 
 
@@ -454,6 +459,7 @@ export class Router {
 }
 
 export class RootRouter extends Router {
+  /** @internal */
   _location: Location;
 
   constructor(registry: RouteRegistry, location: Location, primaryComponent: Type) {

modules/angular2/src/router/sync_route_handler.ts

@@ -3,6 +3,7 @@ import {Promise, PromiseWrapper} from 'angular2/src/core/facade/async';
 import {Type} from 'angular2/src/core/facade/lang';
 
 export class SyncRouteHandler implements RouteHandler {
+  /** @internal */
   _resolvedComponent: Promise<any> = null;
 
   constructor(public componentType: Type, public data?: Object) {

modules/angular2/src/router/url_parser.ts

@@ -22,6 +22,7 @@ export class Url {
 
   segmentToString(): string { return this.path + this._matrixParamsToString(); }
 
+  /** @internal */
   _auxToString(): string {
     return this.auxiliary.length > 0 ?
                ('(' + this.auxiliary.map(sibling => sibling.toString()).join('//') + ')') :
@@ -36,6 +37,7 @@ export class Url {
     return ';' + serializeParams(this.params).join(';');
   }
 
+  /** @internal */
   _childString(): string { return isPresent(this.child) ? ('/' + this.child.toString()) : ''; }
 }