Use prebuilt dart to generate strong sdk summary
Use prebuilt dart to generate strong sdk summary

This saves ~5 minutes on Flutter Engine linux arm64 cross-builds.

Change-Id: I67d849422886101c217de7c2f21bc4eba4f6706a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/201042
Reviewed-by: Alexander Aprelev <dfb74c35a357c2059971a4bf7fe26b6a7c18085d@google.com>
Commit-Queue: Zach Anderson <9f8b31e960a68dade71618f1ff6da6e23a1edfc9@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk