Restore tests/corelib_strong/sort_helper.dart
Restore tests/corelib_strong/sort_helper.dart

This file was moved instead of copied in edfb6d02bfd69a9ad8867fe9e401414da246d2cb, but it is still required.

R=dmitryas@google.com

Review-Url: https://codereview.chromium.org/2986883002 .

dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk