[DDC] Failures in end-to-end expression compiler tests

[fishythefish]

pkg/dev_compiler/test/expression_compiler/expression_compiler_e2e_ddc_unsound_test is failing on unittest-asserts-debug-linux and unittest-asserts-release-mac-arm64. (It went from flaky to timeout on the former rather than failing with a runtime error.)

pkg/dev_compiler/test/expression_compiler/expression_compiler_e2e_amd_sound_test, pkg/dev_compiler/test/expression_compiler/expression_compiler_e2e_amd_unsound_test, and pkg/dev_compiler/test/expression_compiler/expression_compiler_e2e_ddc_sound_test are all failing on unittest-asserts-release-mac and unittest-asserts-release-mac-arm64.

[annagrin]

Thanks @fishythefish! There are couple of issues causing the failures:

• CFE change to improve the switch statements for patterns breaks incorrect test expectations in expression compiler e2e tests.
  04e6f4c

• There seems to be an issue detecting the end of a scope for blocks inside switch statements due to synthesized variables that appear before user block but have file offsets from later code.

Fix: https://dart-review.googlesource.com/c/sdk/+/290706