Flutter: Cannot use Firestore.instance even after importing the package(cloud_firestore)

Error Image:https://i.stack.imgur.com/SZLOO.png

I cannot use Firestore.instance even after importing the required package(cloud_firestore).I have uploaded the image showing the error.Please consider helping.

I have imported the following in pubspec.yaml

name: fluttershare
description: A new Flutter project.


publish_to: 'none' 


version: 1.0.0+1

environment:
  sdk: ">=2.7.0 <3.0.0"

dependencies:
  flutter:
    sdk: flutter
  cloud_firestore: ^0.16.0+1
  image_picker: ^0.6.7+22
  firebase_core: ^0.7.0
  firebase_storage: ^7.0.0
  firebase_auth: ^0.20.0+1
  google_sign_in: ^4.5.9
  geolocator: ^6.2.0
  uuid: ^2.2.2
  image: ^2.1.19
  animator: ^2.0.2
  path_provider: ^1.6.27
  firebase_messaging: ^6.0.16
  timeago: ^2.0.29
  cached_network_image:
  flutter_svg:
  cupertino_icons: ^1.0.2

dev_dependencies:
  flutter_test:
    sdk: flutter


flutter:
  uses-material-design: true
  fonts:
    - family: Signatra
      fonts:
        - asset: assets/fonts/Signatra.ttf
  assets:
    - assets/images/google_signin_button.png
    - assets/images/upload.svg
    - assets/images/search.svg
    - assets/images/activity_feed.svg
    - assets/images/no_content.svg