Ignore unnecessary_import reports in sample code (#84125)

dev/bots/analyze_sample_code.dart

@@ -265,6 +265,7 @@ class SampleChecker {
     return _headers ??= <String>[
       '// generated code',
       '// ignore_for_file: directives_ordering',
+      '// ignore_for_file: unnecessary_import',
       '// ignore_for_file: unused_import',
       '// ignore_for_file: unused_element',
       '// ignore_for_file: unused_local_variable',