Fix navigation scrolling with big projects

Change-Id: Ie5fb3f79225354aef956e71467c0fe34b5e4cbc2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/168122
Reviewed-by: Devon Carew <devoncarew@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

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

@@ -250,7 +250,7 @@ footer .sdk-version {
 }
 
 .nav-inner ul {
-  max-height: 2000px;
+  /* a max-height is added to each element at runtime. */
   transition: max-height 0.5s ease-out;
 }