Tests for --gen json
Tests for --gen json

Summary:
[Thrift] Tests for `--gen json`.

Includes a repro of a bug affecting generated json when a field is seen with a type not yet seen in the program, even if that type is seen later in the program.

Reviewed By: eduardo-elizondo

Differential Revision: D5026376

fbshipit-source-id: f80d7d5665f8faf8c67b8f7772a70693c995c1b8

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