graphql: restore ws test by introducing a ws echo server written in dart

[vincenzopalazzo]

Fixes #980

[codecov]

Codecov Report

Merging #1010 (eb0b83d) into beta (a736d02) will increase coverage by 19.26%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             beta    #1010       +/-   ##
===========================================
+ Coverage   31.88%   51.15%   +19.26%     
===========================================
  Files           8       36       +28     
  Lines         207     1345     +1138     
===========================================
+ Hits           66      688      +622     
- Misses        141      657      +516     

Flag	Coverage Δ
graphql_client	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/graphql/lib/src/core/query_result.dart	48.00% <0.00%> (ø)
packages/graphql/lib/src/cache/fragment.dart	20.45% <0.00%> (ø)
packages/graphql/lib/src/core/fetch_more.dart	0.00% <0.00%> (ø)
...raphql/lib/src/cache/_optimistic_transactions.dart	78.94% <0.00%> (ø)
packages/graphql/lib/src/utilities/helpers.dart	91.30% <0.00%> (ø)
packages/graphql/lib/src/core/_data_class.dart	16.66% <0.00%> (ø)
...b/src/links/websocket_link/websocket_messages.dart	47.50% <0.00%> (ø)
...ackages/graphql/lib/src/core/observable_query.dart	42.37% <0.00%> (ø)
packages/graphql/lib/src/cache/store.dart	100.00% <0.00%> (ø)
...es/graphql/lib/src/core/_query_write_handling.dart	65.38% <0.00%> (ø)
... and 18 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a736d02...eb0b83d. Read the comment docs.