Bump SDK constraint and add dependency on fixnum (#377)

Author: mraleph

CHANGELOG.md

@@ -1,6 +1,10 @@
 ## 2.7.0
 
-* Added decoding/parsing of `grpc-status-details-bin` to pass all response exception details to the `GrpcError` thrown in Dart, via [#349](https://github.com/grpc/grpc-dart/pull/349).
+* Added decoding/parsing of `grpc-status-details-bin` to pass all response
+  exception details to the `GrpcError` thrown in Dart, via
+  [#349](https://github.com/grpc/grpc-dart/pull/349).
+* Dart SDK constraint is bumped to `>=2.3.0 <3.0.0` due to language version
+  in the generated protobuf code.
 
 ## 2.6.0
 

pubspec.yaml

@@ -3,15 +3,15 @@ description: Dart implementation of gRPC, a high performance, open-source univer
 
 version: 2.7.0
 
-author: Dart Team <[email protected]>
 homepage: https://github.com/dart-lang/grpc-dart
 
 environment:
-  sdk: ">=2.2.0 <3.0.0"
+  sdk: '>=2.3.0 <3.0.0'
 
 dependencies:
   async: ^2.2.0
   crypto: ^2.1.4
+  fixnum: ^0.10.11
   googleapis_auth: ^0.2.7
   meta: ^1.1.6
   http: ^0.12.0