Remove workaround now that type promotion accounts for local boolean variables. (#74449)

packages/flutter/lib/src/widgets/scrollable.dart

@@ -982,16 +982,6 @@ class ScrollAction extends Action<ScrollIntent> {
     final bool contextIsValid = focus != null && focus.context != null;
     if (contextIsValid) {
       // Check for primary scrollable within the current context
-      // After https://github.com/dart-lang/language/issues/1274 is implemented,
-      // `focus` will be promoted to non-nullable so we won't need to null check
-      // it (and it will cause a build failure to try to do so).  Until then, we
-      // need to null check it in a way that won't cause a build failure once
-      // the feature is implemented.  We can do that using an explicit "if"
-      // test.
-      // TODO(paulberry): remove this hack once the feature is implemented.
-      if (focus == null) { // ignore: dead_code
-        throw 'This throw is unreachable';
-      }
       if (Scrollable.of(focus.context!) != null)
         return true;
       // Check for fallback scrollable with context from PrimaryScrollController