[nnbd_migration_tool] Quicker file-list ease-out

Half a second was becoming tedious as my current workflow means I
collapse 20 directories each time I re-run from source.

I'd love the tree to retain its state with re-runs, but that is more
work.

Change-Id: Ie53a535a05b445056f87b80131f96f78ae98de2d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/245562
Commit-Queue: Stephen Adams <sra@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>

pkg/nnbd_migration/lib/src/front_end/resources/migration.css

@@ -267,7 +267,7 @@ footer .sdk-version {
 
 .nav-inner ul {
   /* a max-height is added to each element at runtime. */
-  transition: max-height 0.5s ease-out;
+  transition: max-height 0.25s ease-out;
 }
 
 .nav-inner ul.collapsed {