[dart] Remove deprecated avoid_as lint rule

See: https://dart-lang.github.io/linter/lints/avoid_as.html
Change-Id: I7ea4a4071a49a89e2aa30fafd880ab16c8df0fc7
Reviewed-on: https://fuchsia-review.googlesource.com/c/topaz/+/518640
Fuchsia-Auto-Submit: Sanjay Chouksey <sanjayc@google.com>
Reviewed-by: Naud Ghebre <naudzghebre@google.com>
Reviewed-by: Chase Latta <chaselatta@google.com>
Commit-Queue: Sanjay Chouksey <sanjayc@google.com>
diff --git a/tools/analysis_options.yaml b/tools/analysis_options.yaml
index e006f87..aed5d17 100644
--- a/tools/analysis_options.yaml
+++ b/tools/analysis_options.yaml
@@ -23,7 +23,7 @@
     - annotate_overrides
     # Actively discouraged with Dart 2.
     # - avoid_annotating_with_dynamic
-    - avoid_as
+    # - avoid_as
     - avoid_bool_literals_in_conditional_expressions
     - avoid_catches_without_on_clauses
     - avoid_catching_errors