[node_preamble] Update pubspec to 3.8 and reformat.

Change-Id: I8e61375dc29668d279a9f1bef982e4597d35c68e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/427581
Auto-Submit: Mayank Patke <fishythefish@google.com>
Commit-Queue: Sigmund Cherem <sigmund@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>

pkg/node_preamble/lib/preamble.dart

@@ -2,7 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-String getPreamble({bool minified = false}) =>
-    throw UnsupportedError(
-      'Stub only, this package is not supported in the Dart SDK',
-    );
+String getPreamble({bool minified = false}) => throw UnsupportedError(
+  'Stub only, this package is not supported in the Dart SDK',
+);

pkg/node_preamble/pubspec.yaml

@@ -1,7 +1,7 @@
 # This is just a stub, not the real node_preamble package
 name: node_preamble
 environment:
-  sdk: ^3.7.0
+  sdk: ^3.8.0
 
 resolution: workspace