[typescript-fetch] Remove a cycle in the discriminator dependency graph.

[jyasskin]

This is very similar to #6140 and came up in GoogleChrome/chromium-dashboard#3819. I haven't reduced a test case from that PR, but I can if the maintainers here want. I did check that this change removes the circular dependency from GoogleChrome/chromium-dashboard#3819, although getting rid of the cycle requires writing the schema in a way that removes the inheritance from the common base class.

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