Fix analyzer warning from test for issue21079.

tests/language/issue21079_test.dart

Index: tests/language/issue21079_test.dart
diff --git a/tests/language/issue21079_test.dart b/tests/language/issue21079_test.dart
index f0af3f5c97e625ac03e0e08dcfc23574b971af2d..e41c46e5c5f553f7f6ddffcbf324bebe932901e0 100644
--- a/tests/language/issue21079_test.dart
+++ b/tests/language/issue21079_test.dart
@@ -13,6 +13,8 @@ void main() {
   Expect.isTrue(reflectClass(IsolateSpawnException).reflectedType == IsolateSpawnException);
 }
 
-class MyException extends IsolateSpawnException {}
+class MyException extends IsolateSpawnException {
+  MyException() : super("Test") {}
+}