Remove `PbMap._checkNotNull`, use `ArgumentError.checkNotNull` (#708)
Remove `PbMap._checkNotNull`, use `ArgumentError.checkNotNull` (#708)

This function exists in standard library. Remove the copy in `PbMap.`
google/protobuf.dart,google/protobuf.dart,dart-lang/dart-protobuf,dart-lang/dart-protobuf,google/protobuf.dart,dart-lang/dart-protobuf,dart-lang/protobuf,dart-lang/protobuf,dart-lang/protobuf