Answers for "Error: MissingPluginException(No implementation found for method Query#observe on channel plugins.flutter.io/firebase_database) web"

MissingPluginException(No implementation found for method DocumentReference#setData on channel plugins.flutter.io/cloud_firestore

flutter clean
flutter packages get

MissingPluginException(No implementation found for method share on channel flutter_share)

The share plugin works as intended. The error probably appears because
the addition of the plugin was followed by hot restart or hot reload
rather than full restart. This means the plugin's platform-specific code
is not built into the app.

Work around: stop the app and restart it after adding a plugin.

Documentation PR: flutter/website#1038