Enforce the resolution of the current pubspec.lock

Fail the `pub get` with an error message if the pubspec.lock does not
exactly specify a valid resolution of pubspec.yaml or if any content
hash of a hosted package has changed.

Author: elasticdog

.github/workflows/check-flutter.yml

@@ -17,7 +17,7 @@ jobs:
           flutter-version-file: pubspec.yaml
           cache: true
       - name: Install dependencies
-        run: flutter pub get
+        run: flutter pub get --enforce-lockfile
       - name: Check formatting
         run: dart format --output=none --set-exit-if-changed .
 
@@ -34,7 +34,7 @@ jobs:
           flutter-version-file: pubspec.yaml
           cache: true
       - name: Install dependencies
-        run: flutter pub get
+        run: flutter pub get --enforce-lockfile
       - name: Check lints
         run: flutter analyze
 
@@ -51,6 +51,6 @@ jobs:
           flutter-version-file: pubspec.yaml
           cache: true
       - name: Install dependencies
-        run: flutter pub get
+        run: flutter pub get --enforce-lockfile
       - name: Run tests
         run: flutter test