我已经尝试过flutter clean和flutter pub get，这似乎是类似帖子的主要建议，其他一些建议编辑一个我没有GeneratedPluginRegistrant.m的文件，我有一个.java的，但它看起来不像例子，它说得很清楚，它不应该被编辑。
AndroidManifest.xml

...
<application>
...
<meta-data android:name="com.facebook.sdk.ApplicationId" 
        android:value="@string/facebook_app_id"/>
             <meta-data
            android:name="com.google.android.gms.ads.APPLICATION_ID"
            android:value="ca-app-pub-xxxx"/>
</application>

main.dart

...
import 'package:google_mobile_ads/google_mobile_ads.dart';

...
  WidgetsFlutterBinding.ensureInitialized();

  await Firebase.initializeApp(
    options: DefaultFirebaseOptions.currentPlatform,
  );

  await MobileAds.instance.initialize();
...

完整错误：

Uncaught (in promise) Error: MissingPluginException(No implementation found for method _init on channel plugins.flutter.io/google_mobile_ads)
    at Object.throw_ [as throw] (errors.dart:251:49)
    at platform_channel.MethodChannel.new._invokeMethod (platform_channel.dart:165:7)
    at _invokeMethod.next (<anonymous>)
    at async_patch.dart:45:50
    at _RootZone.runUnary (zone.dart:1685:54)
    at _FutureListener.thenAwait.handleValue (future_impl.dart:147:18)
    at handleValueCallback (future_impl.dart:766:44)
    at _Future._propagateToListeners (future_impl.dart:795:13)
    at [_completeWithValue] (future_impl.dart:566:5)
    at async._AsyncCallbackEntry.new.callback (future_impl.dart:639:7)
    at Object._microtaskLoop (schedule_microtask.dart:40:11)
    at _startMicrotaskLoop (schedule_microtask.dart:49:5)
    at async_patch.dart:166:15
throw_ @ errors.dart:251
_invokeMethod @ platform_channel.dart:165
(anonymous) @ async_patch.dart:45
runUnary @ zone.dart:1685
handleValue @ future_impl.dart:147
handleValueCallback @ future_impl.dart:766
_propagateToListeners @ future_impl.dart:795
[_completeWithValue] @ future_impl.dart:566
(anonymous) @ future_impl.dart:639
_microtaskLoop @ schedule_microtask.dart:40
_startMicrotaskLoop @ schedule_microtask.dart:49
(anonymous) @ async_patch.dart:166
Promise.then (async)
_scheduleImmediateWithPromise @ async_patch.dart:164
_scheduleImmediate @ async_patch.dart:136
_scheduleAsyncCallback @ schedule_microtask.dart:69
_rootScheduleMicrotask @ zone.dart:1493
scheduleMicrotask @ zone.dart:1705
[_asyncCompleteWithValue] @ future_impl.dart:638
[_asyncComplete] @ future_impl.dart:598
complete @ future_impl.dart:36
(anonymous) @ firebase_core_web.dart:179
wrapper @ js_patch.dart:356
execCb @ require.js:1696
check @ require.js:883
(anonymous) @ require.js:1139
(anonymous) @ require.js:134
(anonymous) @ require.js:1189
each @ require.js:59
emit @ require.js:1188
check @ require.js:938
enable @ require.js:1176
init @ require.js:788
callGetModule @ require.js:1203
completeLoad @ require.js:1590
onScriptLoad @ require.js:1717
load (async)
req.load @ require.js:1942
load @ require.js:1685
load @ require.js:834
fetch @ require.js:824
check @ require.js:856
enable @ require.js:1176
enable @ require.js:1557
(anonymous) @ require.js:1161
(anonymous) @ require.js:134
each @ require.js:59
enable @ require.js:1113
init @ require.js:788
(anonymous) @ require.js:1460
setTimeout (async)
req.nextTick @ require.js:1815
localRequire @ require.js:1449
requirejs @ require.js:1797
callMethod @ js_patch.dart:143
_initializeCoreRequireJs @ firebase_core_web.dart:175
runBody @ async_patch.dart:84
_async @ async_patch.dart:123
[_initializeCoreRequireJs] @ firebase_core_web.dart:123
initializeApp @ firebase_core_web.dart:205
runBody @ async_patch.dart:84
_async @ async_patch.dart:123
initializeApp @ firebase_core_web.dart:198
initializeApp @ firebase.dart:40
runBody @ async_patch.dart:84
_async @ async_patch.dart:123
initializeApp @ firebase.dart:36
main$ @ main.dart:32
runBody @ async_patch.dart:84
_async @ async_patch.dart:123
main$ @ main.dart:29
(anonymous) @ web_entrypoint.dart:19
_checkAndCall @ operations.dart:334
dcall @ operations.dart:339
(anonymous) @ initialization.dart:75
(anonymous) @ async_patch.dart:45
runUnary @ zone.dart:1685
handleValue @ future_impl.dart:147
handleValueCallback @ future_impl.dart:766
_propagateToListeners @ future_impl.dart:795
[_completeWithValue] @ future_impl.dart:566
(anonymous) @ future_impl.dart:639
_microtaskLoop @ schedule_microtask.dart:40
_startMicrotaskLoop @ schedule_microtask.dart:49
(anonymous) @ async_patch.dart:166
Promise.then (async)
_scheduleImmediateWithPromise @ async_patch.dart:164
_scheduleImmediate @ async_patch.dart:136
_scheduleAsyncCallback @ schedule_microtask.dart:69
_rootScheduleMicrotask @ zone.dart:1493
scheduleMicrotask @ zone.dart:1705
[_asyncCompleteWithValue] @ future_impl.dart:638
[_asyncComplete] @ future_impl.dart:598
runBody @ async_patch.dart:108
_async @ async_patch.dart:123
initializeEngineUi @ initialization.dart:223
(anonymous) @ initialization.dart:73
runBody @ async_patch.dart:84
_async @ async_patch.dart:123
(anonymous) @ initialization.dart:69
_checkAndCall @ operations.dart:334
callMethod @ operations.dart:398
dsend @ operations.dart:402
(anonymous) @ app_bootstrap.dart:72
runBody @ async_patch.dart:84
_async @ async_patch.dart:123
(anonymous) @ app_bootstrap.dart:70
_checkAndCall @ operations.dart:334
dcall @ operations.dart:339
ret @ js_patch.dart:413
(anonymous) @ app_bootstrap.dart:68
_checkAndCall @ operations.dart:334
dcall @ operations.dart:339
ret @ js_patch.dart:413
(anonymous) @ (index):57
Show 34 more frames
errors.dart:251 
        
       Uncaught (in promise) Error: MissingPluginException(No implementation found for method MobileAds#initialize on channel plugins.flutter.io/google_mobile_ads)
    at Object.throw_ [as throw] (errors.dart:251:49)
    at platform_channel.MethodChannel.new._invokeMethod (platform_channel.dart:165:7)
    at _invokeMethod.next (<anonymous>)
    at async_patch.dart:45:50
    at _RootZone.runUnary (zone.dart:1685:54)
    at _FutureListener.thenAwait.handleValue (future_impl.dart:147:18)
    at handleValueCallback (future_impl.dart:766:44)
    at _Future._propagateToListeners (future_impl.dart:795:13)
    at [_completeWithValue] (future_impl.dart:566:5)
    at async._AsyncCallbackEntry.new.callback (future_impl.dart:639:7)
    at Object._microtaskLoop (schedule_microtask.dart:40:11)
    at _startMicrotaskLoop (schedule_microtask.dart:49:5)
    at async_patch.dart:166:15