fix(firebase_performance): fix the token verification. (#6912)

This issue causes the example app to throw the error "'Platform interfaces must not be implemented with `implements`'" from PlatformInterface.verifyToken function call when running on the web browser. It will also cause the web plugin unit test (coming in the next PR) to fail due to the same error.

Author: yalunqin

packages/firebase_performance/firebase_performance/README.md

@@ -93,6 +93,10 @@ To contribute a change to this plugin,
 please review our [contribution guide](https://github.com/FirebaseExtended/flutterfire/blob/master/CONTRIBUTING.md)
 and open a [pull request](https://github.com/FirebaseExtended/flutterfire/pulls).
 
+### Testing
+
+The unit test is in `test` directory which you can run using `flutter test`.
+
 The integration test is in `example/test_driver/firebase_performance_e2e.dart` which you can run on an emulator:
 ```
 cd example

packages/firebase_performance/firebase_performance/pubspec.yaml

@@ -14,7 +14,7 @@ environment:
 dependencies:
   firebase_core: ^1.6.0
   firebase_core_platform_interface: ^4.0.1
-  firebase_performance_platform_interface: ^0.0.1+1
+  firebase_performance_platform_interface: ^0.0.1+2
   flutter:
     sdk: flutter
 

packages/firebase_performance/firebase_performance_platform_interface/CHANGELOG.md

@@ -1,3 +1,8 @@
+## 0.0.1+2
+
+  **FIX**: fix the token verification (#6912).
+
+
 ## 0.0.1+1
 
  - **REFACTOR**: create interface plugin that federates the main plugin (#6851).

packages/firebase_performance/firebase_performance_platform_interface/lib/src/platform_interface/platform_interface_firebase_performance.dart

@@ -22,9 +22,10 @@ enum HttpMethod { Connect, Delete, Get, Head, Options, Patch, Post, Put, Trace }
 /// [FirebasePerformancePlatform] methods.
 abstract class FirebasePerformancePlatform extends PlatformInterface {
   /// Create an instance using [app].
-  FirebasePerformancePlatform({this.appInstance}) : super(token: Object());
+  FirebasePerformancePlatform({this.appInstance}) : super(token: _token);
 
   static FirebasePerformancePlatform? _instance;
+  static final Object _token = Object();
 
   /// The current default [FirebasePerformancePlatform] instance.
   ///
@@ -36,7 +37,7 @@ abstract class FirebasePerformancePlatform extends PlatformInterface {
 
   /// Sets the [FirebasePerformancePlatform] instance.
   static set instance(FirebasePerformancePlatform instance) {
-    PlatformInterface.verifyToken(instance, Object());
+    PlatformInterface.verifyToken(instance, _token);
     _instance = instance;
   }
 

packages/firebase_performance/firebase_performance_platform_interface/pubspec.yaml

@@ -1,6 +1,6 @@
 name: firebase_performance_platform_interface
 description:  A common platform interface for the firebase_performance plugin.
-version: 0.0.1+1
+version: 0.0.1+2
 homepage: https://firebase.flutter.dev/docs/performance/overview
 
 environment: