Update pinned SDK to 3.5.0-278.0.dev

Change-Id: I668fd8b0fde215f53dec3c69b3f5b356c97ae5e6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/372921
Reviewed-by: Matan Lurey <matanl@google.com>
Commit-Queue: Matan Lurey <matanl@google.com>
Auto-Submit: Ben Konyi <bkonyi@google.com>

DEPS

diff --git a/DEPS b/DEPS
index 62e115d..d5cd11a 100644
--- a/DEPS
+++ b/DEPS
@@ -50,7 +50,7 @@
   # Checked-in SDK version. The checked-in SDK is a Dart SDK distribution
   # in a cipd package used to run Dart scripts in the build and test
   # infrastructure, which is automatically built on the release commits.
-  "sdk_tag": "version:3.4.0-247.0.dev",
+  "sdk_tag": "version:3.5.0-278.0.dev",
 
   # co19 is a cipd package automatically generated for each co19 commit.
   # Use tests/co19/update.sh to update this hash.