Fix unnecessary_parenthesis with postfix bang operator. (dart-archive/linter#3904)

dart-lang/linter#3848

Author: asashour

lib/src/rules/unnecessary_parenthesis.dart

@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:analyzer/dart/ast/ast.dart';
+import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 
 import '../analyzer.dart';
@@ -104,6 +105,9 @@ class _Visitor extends SimpleAstVisitor<void> {
           // Code like `(String).noSuchMethod()` is allowed.
           return;
         }
+      } else if (parent is PostfixExpression &&
+          parent.operator.type == TokenType.BANG) {
+        return;
       }
       rule.reportLint(node);
       return;

test_data/rules/unnecessary_parenthesis.dart

@@ -120,6 +120,11 @@ main() async {
   (a?.abs()).hashCode;
   (a?..abs()).hashCode;
   (a?[0]).hashCode;
+
+  (a?.sign)!;
+  (a?.abs())!;
+  (a?..abs())!;
+  (a?[0])!;
 }
 
 Invocation? invocation() => null;