Fix new unnecessary_parenthesis diagnostics. (flutter#38291)

These will get flagged w/ the latest linter.

See, [try failure](https://logs.chromium.org/logs/dart/buildbucket/cr-buildbucket/8794866247739072257/+/u/analyze_flutter_plugins/stdout) and [linter roll PR](https://dart-review.googlesource.com/c/sdk/+/275044).

Author: pq

testing/dart/image_dispose_test.dart

@@ -73,17 +73,17 @@ void main() {
     final Image handle1 = frame.image.clone();
     final Image handle2 = handle1.clone();
 
-    List<StackTrace> stackTraces = (frame.image.debugGetOpenHandleStackTraces())!;
+    List<StackTrace> stackTraces = frame.image.debugGetOpenHandleStackTraces()!;
     expect(stackTraces.length, 3);
     expect(stackTraces, equals(handle1.debugGetOpenHandleStackTraces()));
 
     handle1.dispose();
-    stackTraces = (frame.image.debugGetOpenHandleStackTraces())!;
+    stackTraces = frame.image.debugGetOpenHandleStackTraces()!;
     expect(stackTraces.length, 2);
     expect(stackTraces, equals(handle2.debugGetOpenHandleStackTraces()));
 
     handle2.dispose();
-    stackTraces = (frame.image.debugGetOpenHandleStackTraces())!;
+    stackTraces = frame.image.debugGetOpenHandleStackTraces()!;
     expect(stackTraces.length, 1);
     expect(stackTraces, equals(frame.image.debugGetOpenHandleStackTraces()));