chore(firebase_core): remove dependency on 'universal_platform' (#2916)

Author: mehmetf

packages/firebase_core/firebase_core_platform_interface/lib/firebase_core_platform_interface.dart

@@ -8,11 +8,11 @@ library firebase_core_platform_interface;
 import 'dart:async';
 
 import 'package:flutter/cupertino.dart';
+import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
 import 'package:meta/meta.dart' show required;
 import 'package:plugin_platform_interface/plugin_platform_interface.dart';
 import 'package:quiver/core.dart';
-import 'package:universal_platform/universal_platform.dart';
 
 part 'src/firebase_core_exceptions.dart';
 

packages/firebase_core/firebase_core_platform_interface/lib/src/firebase_core_exceptions.dart

@@ -37,19 +37,19 @@ FirebaseException noDefaultAppInitialization() {
 FirebaseException coreNotInitialized() {
   String message;
 
-  if (UniversalPlatform.isWeb) {
+  if (kIsWeb) {
     message =
         '''Firebase has not been correctly initialized. Have you added the Firebase import scripts to your index.html file? 
     
     View the Web Installation documentation for more information: https://firebaseextended.github.io/flutterfire/docs/installation/web
     ''';
-  } else if (UniversalPlatform.isAndroid) {
+  } else if (defaultTargetPlatform == TargetPlatform.android) {
     message =
         '''Firebase has not been correctly initialized. Have you added the "google-services.json" file to the project? 
     
     View the Android Installation documentation for more information: https://firebaseextended.github.io/flutterfire/docs/installation/android
     ''';
-  } else if (UniversalPlatform.isIOS) {
+  } else if (defaultTargetPlatform == TargetPlatform.iOS) {
     message =
         '''Firebase has not been correctly initialized. Have you added the "GoogleService-Info.plist" file to the project? 
     

packages/firebase_core/firebase_core_platform_interface/pubspec.yaml

@@ -11,7 +11,6 @@ dependencies:
   meta: ^1.1.8
   plugin_platform_interface: ^1.0.2
   quiver: ">=2.0.0 <3.0.0"
-  universal_platform: ^0.1.3
 
 dev_dependencies:
   pedantic: ^1.8.0