protobuf-3.0.0

• Require Dart 2.19.
• Remove ReadonlyMessageMixin (#183, #644)
• Remove frozenMessageModificationHandler (#175, #643)
• Remove PbListBase and FrozenPbList types. All proto repeated fields now use PbList. To check if a list is frozen, use isFrozen getter. (#624, #626)
• Initialize map fields in GeneratedMessage.getField. This behavior is consistent with getField called on repeated fields. (#373, #707)
• Remove unused and optional PbMap constructor argument BuilderInfo? info. (d94d3f0)
• UnknownFieldSetField methods hasRequiredFields, isInitialized and getter length removed. (#721)
• Update library documentation to hide internals, add documentation for public types. (#681)
• Fix PbMap._isReadonly field initialization in PbMap.unmodifiable. (#741, #754)
• Fix decoding map fields when key or value (or both) fields of a map entry is missing. (#719, #745)
• Fix updating frozen (immutable) messages with merge methods (mergeFromBuffer, mergeFromProto3Json, ...). (#489, #727)
• Fix handling null values in proto3 JSON deserializer. (#751, #760, #763)
• Fix handling negative JSON values when parsing uint32 fields. (#839)
• Avoid serializing unknown fields twice in reparseMessage. (#840)