Reduce strong mode errors and warnings

pkg/front_end/.analysis_options

Index: pkg/front_end/.analysis_options
diff --git a/pkg/front_end/.analysis_options b/pkg/front_end/.analysis_options
index f0ac32f0e1ec5d082651f9b218c926396ba7adf8..9a5216af285e6e97eaffc1b4a6ad96b034341000 100644
--- a/pkg/front_end/.analysis_options
+++ b/pkg/front_end/.analysis_options
@@ -2,3 +2,19 @@ analyzer:
   strong-mode: true
   language:
     enableSuperMixins: true
+  errors:
+    # Omits warnigns due to missing `@virtual`
+    # TODO(sigmund): enable once #28601 is fixed
+    strong_mode_invalid_field_override: ignore
+
+    # Omits warnings for implicit casts with generics like `List<T> x = list;`,
+    # also removes warnings due to a bad interaction between inference of
+    # guarded type arguments and downward cast warnings (see #28646).
+    strong_mode_down_cast_composite: ignore
+
+    # Allow having TODOs in the code
+    todo: ignore
+
+    # Allow deprecated calls (althout it would be nice to have a distinction
+    # between internal and external deprecated calls).
+    deprecated_member_use: ignore