[infra] Add --no-preview-dart-2 flag to test.py to prepare for flag flip
[infra] Add --no-preview-dart-2 flag to test.py to prepare for flag flip

This flag is a no-op until this change lands:
https://dart-review.googlesource.com/c/sdk/+/58240

Change-Id: I6f0fcd053dc84738959b67a628d17165e12d3fac
Reviewed-on: https://dart-review.googlesource.com/59741
Reviewed-by: Siva Annamalai <2e618a8cbd7bb6b3d4409061ae407d692a098546@google.com>

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