Use Uint32List as the return type of _IntervalListBuilder.buildIntervalList().

pkg/kernel/lib/class_hierarchy.dart

Index: pkg/kernel/lib/class_hierarchy.dart
diff --git a/pkg/kernel/lib/class_hierarchy.dart b/pkg/kernel/lib/class_hierarchy.dart
index bd2398d8ebe88a5b63885b33000e34688527f8d0..8388fc481b22878102bd863a59b9f44cb2e31fbc 100644
--- a/pkg/kernel/lib/class_hierarchy.dart
+++ b/pkg/kernel/lib/class_hierarchy.dart
@@ -823,7 +823,7 @@ class _IntervalListBuilder {
     }
   }
 
-  List<int> buildIntervalList() {
+  Uint32List buildIntervalList() {
     // Sort the event points and sweep left to right while tracking how many
     // intervals we are currently inside.  Record an interval end point when the
     // number of intervals drop to zero or increase from zero to one.