fix(aio): move interfaces into their own files to workaround compile weirdness

For some reason the tree-shaker is not picking up these interfaces
(perhaps TS is not passing it through) when they are in the same file
as their related services. This results in a distracting warning message.

aio/src/app/documents/document-contents.ts

@@ -0,0 +1,4 @@
+export interface DocumentContents {
+  title: string;
+  contents: string;
+}

aio/src/app/documents/document.service.ts

@@ -7,14 +7,11 @@ import 'rxjs/add/operator/switchMap';
 
 import { LocationService } from 'app/shared/location.service';
 import { Logger } from 'app/shared/logger.service';
+import { DocumentContents } from './document-contents';
+export { DocumentContents } from './document-contents';
 
 const FILE_NOT_FOUND_URL = 'file-not-found';
 
-export interface DocumentContents {
-  title: string;
-  contents: string;
-}
-
 @Injectable()
 export class DocumentService {
 

aio/src/app/navigation/navigation-node.ts

@@ -0,0 +1,7 @@
+export interface NavigationNode {
+  url?: string;
+  title?: string;
+  tooltip?: string;
+  target?: string;
+  children?: NavigationNode[];
+}

aio/src/app/navigation/navigation.service.ts

@@ -7,13 +7,8 @@ import 'rxjs/add/operator/publish';
 import { Logger } from 'app/shared/logger.service';
 import { LocationService } from 'app/shared/location.service';
 
-export interface NavigationNode {
+import { NavigationNode } from './navigation-node';
-  url?: string;
+export { NavigationNode } from './navigation-node';
-  title?: string;
-  tooltip?: string;
-  target?: string;
-  children?: NavigationNode[];
-}
 
 export interface NavigationViews {
   [name: string]: NavigationNode[];