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/mock/location_mock.ts

@@ -4,8 +4,11 @@ import {Location} from 'angular2/src/router/location';
 
 export class SpyLocation implements Location {
   urlChanges: string[] = [];
+  /** @internal */
   _path: string = '';
+  /** @internal */
   _subject: EventEmitter = new EventEmitter();
+  /** @internal */
   _baseHref: string = '';
 
   setInitialPath(url: string) { this._path = url; }

modules/angular2/src/mock/mock_location_strategy.ts

@@ -7,6 +7,7 @@ export class MockLocationStrategy extends LocationStrategy {
   internalPath: string = '/';
   internalTitle: string = '';
   urlChanges: string[] = [];
+  /** @internal */
   _subject: EventEmitter = new EventEmitter();
   constructor() { super(); }
 

modules/angular2/src/mock/ng_zone_mock.ts

@@ -1,6 +1,7 @@
 import {NgZone} from 'angular2/src/core/zone/ng_zone';
 
 export class MockNgZone extends NgZone {
+  /** @internal */
   _onEventDone: () => void;
 
   constructor() { super({enableLongStackTrace: false}); }

modules/angular2/src/mock/view_resolver_mock.ts

@@ -6,9 +6,13 @@ import {ViewMetadata} from '../core/metadata';
 import {ViewResolver} from 'angular2/src/core/linker/view_resolver';
 
 export class MockViewResolver extends ViewResolver {
+  /** @internal */
   _views = new Map<Type, ViewMetadata>();
+  /** @internal */
   _inlineTemplates = new Map<Type, string>();
+  /** @internal */
   _viewCache = new Map<Type, ViewMetadata>();
+  /** @internal */
   _directiveOverrides = new Map<Type, Map<Type, Type>>();
 
   constructor() { super(); }
@@ -103,6 +107,8 @@ export class MockViewResolver extends ViewResolver {
   }
 
   /**
+   * @internal
+   *
    * Once a component has been compiled, the AppProtoView is stored in the compiler cache.
    *
    * Then it should not be possible to override the component configuration after the component