Migration: Test a complex example of flow analysis integration.
Migration: Test a complex example of flow analysis integration.

At this point I believe the integration between flow analysis and the
migration tool is complete, so I wanted to include a more complex
example at the API test level to demonstrate that it's working.

The example is inspired by discussion at
https://dart-review.googlesource.com/c/sdk/+/112406.

Change-Id: I18152e9a07f4cd8d798cce2ae8908cb7afe6b4be
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/114554
Reviewed-by: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@google.com>

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