Update tools/package_deps to stop at a line with Future with type argument.

Should fix failures in https://dart-review.googlesource.com/c/sdk/+/358222

Change-Id: I1c5d63e90e26e11d083be2327a22f6c5617b6670
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/358280
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Devon Carew <devoncarew@google.com>
diff --git a/tools/package_deps/bin/package_deps.dart b/tools/package_deps/bin/package_deps.dart
index 04dbdcd..b624f92 100644
--- a/tools/package_deps/bin/package_deps.dart
+++ b/tools/package_deps/bin/package_deps.dart
@@ -375,6 +375,7 @@
           line.startsWith('extension ') ||
           line.startsWith('void ') ||
           line.startsWith('Future ') ||
+          line.startsWith('Future<') ||
           line.startsWith('final ') ||
           line.startsWith('const ')) {
         break;