Analyzer: no error assigning to a final local via for in loop

[MichaelRFairhurst]

Discovered seemingly on accident by language_2/void/void_type_usage_test/final_local_for_in2

  final Object x = null;                                                         
  for(x in [1, 2]) {}

This doesn't show any errors but of course should. x is final so it cannot be set to 1 and 2.

[srawlins]

I think I fixed this at some point.

error • The final variable 'x' can only be set once. • 35508.dart:3:7 • assignment_to_final_local