Add isolate extension to readmes (#1390)

Author: marandaneto

dart/README.md

@@ -180,7 +180,7 @@ Read more about [Automatic Instrumentation](https://docs.sentry.io/platforms/dar
 - Use a `catchError` block for `Futures`, examples on [dart.dev](https://dart.dev/guides/libraries/futures-error-handling).
 - The SDK already runs your `callback` on an error handler, e.g. using [runZonedGuarded](https://api.flutter.dev/flutter/dart-async/runZonedGuarded.html), events caught by the `runZonedGuarded` are captured automatically.
 - [Current Isolate errors](https://api.flutter.dev/flutter/dart-isolate/Isolate/addErrorListener.html) which is the equivalent of a main or UI thread, are captured automatically (Only for non-Web Apps).
-- For your own `Isolates`, add an [Error Listener](https://api.flutter.dev/flutter/dart-isolate/Isolate/addErrorListener.html) and call `Sentry.captureException`.
+- For your own `Isolates`, add an [Error Listener](https://api.flutter.dev/flutter/dart-isolate/Isolate/addErrorListener.html) by calling `isolate.addSentryErrorListener()`.
 
 #### Resources
 

flutter/README.md

@@ -123,7 +123,7 @@ Or [try out the Alpha version of the Sentry Dart Plugin](https://github.com/gets
 - Use a `catchError` block for `Futures`, examples on [dart.dev](https://dart.dev/guides/libraries/futures-error-handling).
 - [Flutter-specific errors](https://api.flutter.dev/flutter/foundation/FlutterError/onError.html) are captured automatically.
 - [Current Isolate errors](https://api.flutter.dev/flutter/dart-isolate/Isolate/addErrorListener.html) which is the equivalent of a main or UI thread, are captured automatically (Only for non-Web Apps).
-- For your own `Isolates`, add an [Error Listener](https://api.flutter.dev/flutter/dart-isolate/Isolate/addErrorListener.html) and call `Sentry.captureException`.
+- For your own `Isolates`, add an [Error Listener](https://api.flutter.dev/flutter/dart-isolate/Isolate/addErrorListener.html) by calling `isolate.addSentryErrorListener()`.
 
 #### Resources