[flutter_tools] handle null package (#75336)

packages/flutter_tools/lib/src/dart/language_version.dart

@@ -86,7 +86,7 @@ LanguageVersion determineLanguageVersion(File file, Package package) {
 
   // If the language version cannot be found, use the package version.
   if (package != null) {
-    return package.languageVersion;
+    return package.languageVersion ?? nullSafeVersion;
   }
   // Default to 2.12
   return nullSafeVersion;

packages/flutter_tools/test/general.shard/dart/language_version_test.dart

@@ -254,6 +254,20 @@ library funstuff;
     expect(determineLanguageVersion(file, package), LanguageVersion(2, 7));
   });
 
+  testWithoutContext('defaults to null safe version if package lookup returns null', () {
+    final FileSystem fileSystem = MemoryFileSystem.test();
+    final File file = fileSystem.file('example.dart')
+      ..writeAsStringSync('''
+// Some license
+''');
+    final Package package = Package(
+      'foo',
+      Uri.parse('file://foo/'),
+      languageVersion: null,
+    );
+
+    expect(determineLanguageVersion(file, package), LanguageVersion(2, 12));
+  });
 
   testWithoutContext('Returns null safe error if reading the file throws a FileSystemException', () {
     final Package package = Package(