Fix Flow Analysis Bug in Dart SDK (#60114)

[jhaaryan597]

This PR fixes the incorrect flow analysis after non-nullable type comparison with == null.

• Ensures proper unreachable code detection
• Prevents late variable usage without assignment
  Fixes Wrong flow analysis after non-nulable type and ==null #60114

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[mraleph]

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[mraleph]

Closing as stale as the author did not sign CLA.