hotfix: Opt-out null safety for crypto files because of an error in web

`packages/matrix/src/utils/crypto/crypto.dart:19:1: Error: Null safe libraries are not allowed to export declarations from of opt-out libraries.
`

CHANGELOG.md

@@ -1,4 +1,7 @@
-## [0.3.0] - 2nd Aug 2021
+## [0.3.1] - 20nd Aug 2021
+- hotfix: Opt-out null safety for crypto files because of an error in web
+
+## [0.3.0] - 20nd Aug 2021
 - remove: deprecated moor database (breaking change)
 - feat(events): add plain-text body representation from HTML
 - feat: get new_content in getLocalizedBody

lib/src/utils/crypto/crypto.dart

@@ -1,3 +1,4 @@
+// @dart=2.9
 /*
  *   Famedly Matrix SDK
  *   Copyright (C) 2019, 2020, 2021 Famedly GmbH

lib/src/utils/crypto/ffi.dart

@@ -1,3 +1,4 @@
+// @dart=2.9
 /*
  *   Famedly Matrix SDK
  *   Copyright (C) 2019, 2020, 2021 Famedly GmbH

lib/src/utils/crypto/native.dart

@@ -1,3 +1,4 @@
+// @dart=2.9
 import 'dart:async';
 import 'dart:typed_data';
 import 'dart:ffi';

pubspec.yaml

@@ -1,6 +1,6 @@
 name: matrix
 description: Matrix Dart SDK
-version: 0.3.0
+version: 0.3.1
 homepage: https://famedly.com
 
 environment: