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/core/render/dom/dom_renderer.ts

@@ -195,6 +195,7 @@ export class DomRenderer_ extends DomRenderer {
     return this._componentCmds.get(templateId);
   }
 
+  /** @internal */
   _createRootHostViewScope: WtfScopeFn = wtfCreateScope('DomRenderer#createRootHostView()');
   createRootHostView(hostProtoViewRef: RenderProtoViewRef, fragmentCount: number,
                      hostElementSelector: string): RenderViewWithFragments {
@@ -207,6 +208,7 @@ export class DomRenderer_ extends DomRenderer {
     return wtfLeave(s, this._createView(hostProtoViewRef, element));
   }
 
+  /** @internal */
   _createViewScope = wtfCreateScope('DomRenderer#createView()');
   createView(protoViewRef: RenderProtoViewRef, fragmentCount: number): RenderViewWithFragments {
     var s = this._createViewScope();
@@ -256,6 +258,7 @@ export class DomRenderer_ extends DomRenderer {
     }
   }
 
+  /** @internal */
   _detachFragmentScope = wtfCreateScope('DomRenderer#detachFragment()');
   detachFragment(fragmentRef: RenderFragmentRef) {
     var s = this._detachFragmentScope();

modules/angular2/src/core/render/dom/events/event_manager.ts

@@ -29,6 +29,7 @@ export class EventManager {
 
   getZone(): NgZone { return this._zone; }
 
+  /** @internal */
   _findPluginFor(eventName: string): EventManagerPlugin {
     var plugins = this._plugins;
     for (var i = 0; i < plugins.length; i++) {

modules/angular2/src/core/render/dom/events/key_events.ts

@@ -90,8 +90,8 @@ export class KeyEventsPlugin extends EventManagerPlugin {
     return fullKey;
   }
 
-  static eventCallback(element: HTMLElement, fullKey: any, handler: (e: Event) => any, zone: NgZone):
-      (event: KeyboardEvent) => void {
+  static eventCallback(element: HTMLElement, fullKey: any, handler: (e: Event) => any,
+                       zone: NgZone): (event: KeyboardEvent) => void {
     return (event) => {
       if (StringWrapper.equals(KeyEventsPlugin.getEventFullKey(event), fullKey)) {
         zone.run(() => handler(event));

modules/angular2/src/core/render/dom/shared_styles_host.ts

@@ -5,7 +5,9 @@ import {DOCUMENT} from './dom_tokens';
 
 @Injectable()
 export class SharedStylesHost {
+  /** @internal */
   _styles: string[] = [];
+  /** @internal */
   _stylesSet = new Set<string>();
 
   constructor() {}
@@ -34,6 +36,7 @@ export class DomSharedStylesHost extends SharedStylesHost {
     super();
     this._hostNodes.add(doc.head);
   }
+  /** @internal */
   _addStylesToHost(styles: string[], host: Node) {
     for (var i = 0; i < styles.length; i++) {
       var style = styles[i];