fix(aio): LocationService urls should never start with a slash

There is a weirdness in the Angular Location service.
If the `baseHref` is only a single slash (`'/'`) then it
changes it to be an empty string (`''`). The effect of this
is that `Location.normaliseUrl(url)` does not strip off the
leading slash from url paths.

The problem is that the leading slash only appears on the
initial Location path, and not on urls that arrive from subscribing
to the Location service.

This commit is a workaround this problem.

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

@@ -61,7 +61,7 @@ export class DocumentService {
   }
 
   private computePath(url) {
-    url = url.startsWith('/') ? url : '/' + url;
+    url = '/' + url;
     url = url.endsWith('/') ? url + 'index' : url;
     return 'content/docs' + url + '.json';
   }

aio/src/app/shared/location.service.spec.ts

@@ -32,7 +32,7 @@ describe('LocationService', () => {
 
       let initialUrl;
       service.currentUrl.subscribe(url => initialUrl = url);
-      expect(initialUrl).toEqual('/next-url3');
+      expect(initialUrl).toEqual('next-url3');
     });
 
     it('should emit all location changes after it has been subscribed to', () => {
@@ -51,10 +51,10 @@ describe('LocationService', () => {
       location.simulatePopState('/next-url3');
 
       expect(urls).toEqual([
-        '/initial-url3',
-        '/next-url1',
-        '/next-url2',
-        '/next-url3'
+        'initial-url3',
+        'next-url1',
+        'next-url2',
+        'next-url3'
       ]);
     });
 
@@ -78,15 +78,15 @@ describe('LocationService', () => {
         location.simulatePopState('/next-url3');
 
         expect(urls1).toEqual([
-          '/initial-url3',
-          '/next-url1',
-          '/next-url2',
-          '/next-url3'
+          'initial-url3',
+          'next-url1',
+          'next-url2',
+          'next-url3'
         ]);
 
         expect(urls2).toEqual([
-          '/next-url2',
-          '/next-url3'
+          'next-url2',
+          'next-url3'
         ]);
     });
   });

aio/src/app/shared/location.service.ts

@@ -10,13 +10,22 @@ export class LocationService {
   get currentUrl() { return this.urlSubject.asObservable(); }
 
   constructor(private location: Location) {
-    this.urlSubject = new BehaviorSubject(location.path(true));
-    this.location.subscribe(state => this.urlSubject.next(state.url));
+
+    const initialUrl = this.stripLeadingSlashes(location.path(true));
+    this.urlSubject = new BehaviorSubject(initialUrl);
+
+    this.location.subscribe(state => {
+      const url = this.stripLeadingSlashes(state.url);
+      return this.urlSubject.next(url);
+    });
   }
 
   go(url: string) {
-    url = this.location.normalize(url);
     this.location.go(url);
     this.urlSubject.next(url);
   }
+
+  private stripLeadingSlashes(url: string) {
+    return url.replace(/^\/+/, '');
+  }
 }