Enable `unnecessary_null_comparison` check (#118849)

* Enable unnecessary_null_comparison outside of packages

* fix snippet

* enable for all

analysis_options.yaml

@@ -23,8 +23,6 @@ analyzer:
     # allow self-reference to deprecated members (we do this because otherwise we have
     # to annotate every member in every test, assert, etc, when we deprecate something)
     deprecated_member_use_from_same_package: ignore
-    # Turned off until null-safe rollout is complete.
-    unnecessary_null_comparison: ignore
   exclude:
     - "bin/cache/**"
       # Ignore protoc generated files

packages/flutter/lib/src/rendering/box.dart

@@ -987,7 +987,6 @@ class _IntrinsicDimensionsCacheEntry {
 /// AxisDirection get axis => _axis;
 /// AxisDirection _axis = AxisDirection.down; // or initialized in constructor
 /// set axis(AxisDirection value) {
-///   assert(value != null); // same checks as in the constructor
 ///   if (value == _axis) {
 ///     return;
 ///   }