keepToString top-level constant

Annotation to keep Object.toString overrides as-is instead of removing them for size optimization purposes.

For certain uris (currently dart:ui and package:flutter) the Dart compiler will remove Object.toString overrides from classes in profile/release mode to reduce code size.

Individual classes can opt out of this behavior via the following annotations:

• @pragma('flutter:keep-to-string')
• @pragma('flutter:keep-to-string-in-subtypes')

See https://github.com/dart-lang/sdk/blob/main/runtime/docs/pragmas.md

For example, in the following class the toString method will remain as return _buffer.toString();, even if the --delete-tostring-package-uri option would otherwise apply and replace it with return super.toString(). (By convention, dart:ui is usually imported as ui, hence the prefix.)

class MyStringBuffer {
  final StringBuffer _buffer = StringBuffer();

  // ...

  @ui.keepToString
  @override
  String toString() {
    return _buffer.toString();
  }
}