[flutter_migrate] Remove one set of parens that wrap a single String literal (flutter#7216)

In order to address dart-lang/linter#4354, we are reporting a few more
cases in `unnecessary_parenthesis`. `('text') * 2` is one such case.
This change is being made in
https://dart-review.googlesource.com/c/sdk/+/376540. (These are the only
newly-reported unnecessary parentheses for this change.)

This is the _same_ change as
flutter#152031

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Author: srawlins

packages/flutter_migrate/test/base/terminal_test.dart

@@ -19,7 +19,7 @@ void main() {
       );
       bufferLogger.printStatus('0123456789' * 8);
 
-      expect(bufferLogger.statusText, equals(('${'0123456789' * 4}\n') * 2));
+      expect(bufferLogger.statusText, equals('${'0123456789' * 4}\n' * 2));
     });
 
     testWithoutContext('can turn off wrapping', () async {