Fix 23 additional lint issues to reduce from 129 to 106 issues

- Add const constructors to @immutable classes (5 issues: prefer_const_constructors_in_immutables)
- Fix nullable value casts in firesApi.dart (3 issues: cast_nullable_to_non_nullable)
- Remove redundant default argument values (2 issues: avoid_redundant_argument_values)
- Make YourLocation immutable with final fields (2 issues: avoid_equals_and_hash_code_on_mutable_classes)
- Update imports and fix formatting

Reduces lint issues from 129 to 106. APK builds successfully (146 MB).

lib/widgets/material_app_with_intro.dart

@@ -1,8 +1,13 @@
 import 'package:flutter/material.dart';
+import 'package:meta/meta.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 import 'dart:async';
 
+@immutable
 abstract class MaterialAppWithIntro extends StatelessWidget {
+  const MaterialAppWithIntro(this.name, this.theme, this.routes,
+      this.introWidget, this.continueWidget, this.prefsKey);
+
   final String name;
   final ThemeData theme;
   final Map<String, WidgetBuilder> routes;
@@ -10,9 +15,6 @@ abstract class MaterialAppWithIntro extends StatelessWidget {
   final WidgetBuilder continueWidget;
   final String prefsKey;
 
-  MaterialAppWithIntro(this.name, this.theme, this.routes, this.introWidget,
-      this.continueWidget, this.prefsKey);
-
   @override
   Widget build(BuildContext context) {
     return MaterialApp(
@@ -25,26 +27,26 @@ abstract class MaterialAppWithIntro extends StatelessWidget {
 }
 
 class MaterialAppWithIntroHome extends StatefulWidget {
+  const MaterialAppWithIntroHome(
+      this.introWidget, this.continueWidget, this.prefsKey);
+
   final WidgetBuilder introWidget;
   final WidgetBuilder continueWidget;
   final String prefsKey;
 
-  const MaterialAppWithIntroHome(
-      this.introWidget, this.continueWidget, this.prefsKey);
-
   @override
   _MaterialAppWithIntroState createState() =>
       _MaterialAppWithIntroState(introWidget, continueWidget, prefsKey);
 }
 
 class _MaterialAppWithIntroState extends State<MaterialAppWithIntroHome> {
+  _MaterialAppWithIntroState(
+      this.introWidget, this.continueWidget, this.prefsKey);
+
   final WidgetBuilder introWidget;
   final WidgetBuilder continueWidget;
   final String prefsKey;
 
-  _MaterialAppWithIntroState(
-      this.introWidget, this.continueWidget, this.prefsKey);
-
   @override
   void initState() {
     super.initState();
@@ -63,19 +65,19 @@ class _MaterialAppWithIntroState extends State<MaterialAppWithIntroHome> {
       await prefs.setBool(initialWizardKey, false);
       if (mounted) {
         await Navigator.of(context)
-            .pushReplacement(MaterialPageRoute(builder: introWidget));
+            .pushReplacement(MaterialPageRoute<void>(builder: introWidget));
       }
     } else {
       if (mounted) {
         await Navigator.of(context)
-            .pushReplacement(MaterialPageRoute(builder: continueWidget));
+            .pushReplacement(MaterialPageRoute<void>(builder: continueWidget));
       }
     }
   }
 
   @override
   Widget build(BuildContext context) {
-    return Scaffold(
+    return const Scaffold(
       body: Center(
         child: CircularProgressIndicator(),
       ),