Patch release for protoc_plugin: bump constraint to use protobuf 4.0.0

Author: mraleph

protoc_plugin/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 22.0.1
+
+* Bump `protobuf` constraint to `^4.0.0`
+
 ## 22.0.0
 
 * Remove `PbEventMixin` mixin. ([#738])

protoc_plugin/pubspec.yaml

@@ -1,5 +1,5 @@
 name: protoc_plugin
-version: 22.0.0
+version: 22.0.1
 description: A protobuf protoc compiler plugin used to generate Dart code.
 repository: https://github.com/google/protobuf.dart/tree/master/protoc_plugin
 
@@ -9,7 +9,7 @@ environment:
 dependencies:
   fixnum: ^1.0.0
   path: ^1.8.0
-  protobuf: ^3.1.0
+  protobuf: ^4.0.0
 
 dev_dependencies:
   collection: ^1.15.0