Bump protobuf from 2.1.0 to 3.1.0 in /dart_sdk

Bumps [protobuf](https://github.com/google/protobuf.dart) from 2.1.0 to 3.1.0.
- [Release notes](https://github.com/google/protobuf.dart/releases)
- [Commits](google/protobuf.dart@protobuf-v2.1.0...protobuf-v3.1.0)

---
updated-dependencies:
- dependency-name: protobuf
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

dart_sdk/pubspec.yaml

@@ -15,7 +15,7 @@ dependencies:
   flutter:
     sdk: flutter
   http: ^0.13.5
-  protobuf: ^2.0.0
+  protobuf: ">=2.0.0 <4.0.0"
   uni_links: ^0.5.1
   uni_links_platform_interface: ^1.0.0
   url_launcher: ^6.1.2