[hhh] Symlink dart sdk and flutter web sdk instead of making a copy.

Saves 15 seconds from the build time.

Change-Id: Ib8c896a3b8b07ec7ae5e34512b6e68e91b4dc62c
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/196549
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>
5 files changed