Support BrowserPlatforms with no window or document

Author: schmidt-sebastian

packages/firestore/src/platform_browser/browser_platform.ts

@@ -27,14 +27,18 @@ export class BrowserPlatform implements Platform {
 
   readonly emptyByteString = '';
 
-  readonly document = document;
-
-  readonly window = window;
-
   constructor() {
     this.base64Available = typeof atob !== 'undefined';
   }
 
+  get document(): Document | null {
+    return typeof document !== 'undefined' ? document : null;
+  }
+
+  get window(): Window | null {
+    return typeof window !== 'undefined' ? window : null;
+  }
+
   loadConnection(databaseInfo: DatabaseInfo): Promise<Connection> {
     return Promise.resolve(new WebChannelConnection(databaseInfo));
   }