fix #25280, treat setters as returning void in strong mode

pkg/analyzer/lib/src/generated/static_type_analyzer.dart

Index: pkg/analyzer/lib/src/generated/static_type_analyzer.dart
diff --git a/pkg/analyzer/lib/src/generated/static_type_analyzer.dart b/pkg/analyzer/lib/src/generated/static_type_analyzer.dart
index 106ff7a72ab22d64c6f9e751b95558ad7414b8e3..ba8aa8cf73d5d7ed4f6a3eff94fd0f86cf9c7b92 100644
--- a/pkg/analyzer/lib/src/generated/static_type_analyzer.dart
+++ b/pkg/analyzer/lib/src/generated/static_type_analyzer.dart
@@ -413,9 +413,11 @@ class StaticTypeAnalyzer extends SimpleAstVisitor<Object> {
     FunctionExpression function = node.functionExpression;
     ExecutableElementImpl functionElement =
         node.element as ExecutableElementImpl;
-    functionElement.returnType =
-        _computeStaticReturnTypeOfFunctionDeclaration(node);
     if (node.parent is FunctionDeclarationStatement) {
+      // TypeResolverVisitor sets the return type for top-level functions, so
+      // we only need to handle local functions.
+      functionElement.returnType =
+          _computeStaticReturnTypeOfFunctionDeclaration(node);
       _recordPropagatedTypeOfFunction(functionElement, function.body);
     }
     _recordStaticType(function, functionElement.type);