Update the pre-built SDK and re-enable the format presubmit.

Now that the new formatter has landed in the SDK, I can update the
pre-built SDK to use it and turn the presubmit back on.

Change-Id: I2ec47dfa2f26f39451b1bf071fdbeb963f19eeaa
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/424282
Commit-Queue: Alexander Thomas <athom@google.com>
Auto-Submit: Bob Nystrom <rnystrom@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>

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": "git_revision:5a8921e0042c662ea2adbac35ea051dec20a5341",
+  "sdk_tag": "git_revision:4bb26ad346b166d759773e01ffc8247893b9681e",
 
   # co19 is a cipd package automatically generated for each co19 commit.
   # Use tests/co19/update.sh to update this hash.

PRESUBMIT.py

@@ -104,10 +104,6 @@ def load_source(modname, filename):
 
 
 def _CheckDartFormat(input_api, output_api):
-    # TODO(rnystrom): Dart format presubmit checks are temporarily disabled
-    # while rolling the new formatter into the SDK.
-    return []
-
     local_root = input_api.change.RepositoryRoot()
     utils = load_source('utils', os.path.join(local_root, 'tools', 'utils.py'))