anchor.diff:    /// function has a return type that Future<Null> is not assignable to.
     ///
     /// See [HintCode.MISSING_RETURN].
  -  void _checkForMissingReturn(TypeAnnotation returnNode, FunctionBody body,
  -      ExecutableElement element, AstNode functionNode) {
  +  void _checkForMissingReturn(FunctionBody body, AstNode functionNode) {
       if (_isNonNullableByDefault) {
         return;
       }
  
positive.code:   @override
    void visitFunctionExpression(FunctionExpression node) {
      if (node.parent is! FunctionDeclaration) {
        _checkForMissingReturn(null, node.body, node.declaredElement, node);
      }
      DartType functionType = InferenceContext.getContext(node);
      if (functionType is! FunctionType) {
  
negative.code:     }
    }
  
    /// In "strict-inference" mode, check that [returnType] is specified.
    void _checkStrictInferenceReturnType(
        AstNode returnType, AstNode reportNode, String displayName) {
      if (!_strictInference) {
  