feat(security): expose the safe value types.

This allows users to properly type their `SafeHtml`, `SafeStyle`, etc values.

Fixes #8568.

modules/@angular/platform-browser/src/platform_browser.ts

@@ -26,7 +26,15 @@ export {
 export * from '../private_export';
 export {DOCUMENT} from './dom/dom_tokens';
 
-export {DomSanitizationService, SecurityContext} from './security/dom_sanitization_service';
+export {
+  DomSanitizationService,
+  SafeHtml,
+  SafeScript,
+  SafeStyle,
+  SafeUrl,
+  SafeResourceUrl,
+  SecurityContext
+} from './security/dom_sanitization_service';
 
 export {
   bootstrapStatic,