Handling of large integers in bytes fields is inconsistent between serializers

[osa1]

Originally reported in #219 (comment)

If I have a bytes field and set the value something like <int>[999999] (i.e. an integer list with integers larger than max value of a byte), binary serializer silently truncates the integer value, but JSON map and proto3 JSON serializers throw an exception.

Exception thrown by JSON map encoder:

Invalid argument (Not a byte value at index 0: 0x1869f): Instance(length:1) of '_GrowableList'
#0      _Base64Encoder.encodeChunk (dart:convert/base64.dart:374:5)
#1      _Base64Encoder.encode (dart:convert/base64.dart:328:9)
#2      Base64Encoder.convert (dart:convert/base64.dart:243:26)
#3      Codec.encode (dart:convert/codec.dart:21:32)
#4      base64Encode (dart:convert/base64.dart:41:48)
#5      _writeToJsonMap.convertToMap (package:protobuf/src/protobuf/json.dart:40:16)
#6      _writeToJsonMap (package:protobuf/src/protobuf/json.dart:79:33)
#7      GeneratedMessage.writeToJsonMap (package:protobuf/src/protobuf/generated_message.dart:203:44)
#8      GeneratedMessage.writeToJson (package:protobuf/src/protobuf/generated_message.dart:218:38)

Exception thrown by proto3 JSON encoder:

Unhandled exception:
Invalid argument (Not a byte value at index 0: 0x1869f): Instance(length:1) of '_GrowableList'
#0      _Base64Encoder.encodeChunk (dart:convert/base64.dart:374:5)
#1      _Base64Encoder.encode (dart:convert/base64.dart:328:9)
#2      Base64Encoder.convert (dart:convert/base64.dart:243:26)
#3      Codec.encode (dart:convert/codec.dart:21:32)
#4      base64Encode (dart:convert/base64.dart:41:48)
#5      _writeToProto3Json.valueToProto3Json (package:protobuf/src/protobuf/proto3_json.dart:77:18)
#6      _writeToProto3Json (package:protobuf/src/protobuf/proto3_json.dart:108:19)
#7      GeneratedMessage.toProto3Json (package:protobuf/src/protobuf/generated_message.dart:235:7)