Update the pinned Dart SDK for workspace support.

Without this, I can't use `workspace` in the Flutter engine:
flutter/engine#53539

Change-Id: I3b68f14b94f91deda329efd98042dcf63ddb80ba
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/373903
Commit-Queue: Ryan Macnak <[email protected]>
Reviewed-by: Ryan Macnak <[email protected]>

Author: matanlurey

DEPS

@@ -50,7 +50,7 @@ vars = {
   # 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:3.5.0-278.0.dev",
+  "sdk_tag": "version:3.5.0-307.0.dev",
 
   # co19 is a cipd package automatically generated for each co19 commit.
   # Use tests/co19/update.sh to update this hash.