Make file Dart 2 compatible.

[pq]

file tests fail due to type errors of the familiar

type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'Map<String, String>' of 'input'

variety.

See: https://travis-ci.org/google/file.dart/jobs/414381120

A few changes get us most of the way there:

PathContextCodec =>

static path.Context _decode(Map<String, dynamic> input) {

and

static const Converter<Map<String, dynamic>, path.Context> deserialize =
      const _ForwardingConverter<Map<String, dynamic>, path.Context>(_decode);

but someone who understands the source should look a little closer.

/cc @tvolkert ?

[tvolkert]

Dup of #88?