Merge changes from internal repo. (#68)

Parsing protobuf from JSON now ignores unknown enum values instead of
throwing an exception.

The behavior is still different from parsing a message from a serialized
byte string, because we do not store the (tag, unknown enum value) pair
in the UnknownFieldSet. JSON parsing currently discards unknown field
tags, so it makes sense to also discard the unknown enum values.

• CHANGELOG.md[diff]
• lib/src/protobuf/json.dart[diff]
• pubspec.yaml[diff]
• test/mock_util.dart[diff]