Optimize subclass/subtype queries

pkg/compiler/lib/src/types/union_type_mask.dart

Index: pkg/compiler/lib/src/types/union_type_mask.dart
diff --git a/pkg/compiler/lib/src/types/union_type_mask.dart b/pkg/compiler/lib/src/types/union_type_mask.dart
index 6f8c81367584a09ac669ef49f8e5eebfb414dc86..61663499171b5bfdd9a94c4b7541b9a857f1bfd4 100644
--- a/pkg/compiler/lib/src/types/union_type_mask.dart
+++ b/pkg/compiler/lib/src/types/union_type_mask.dart
@@ -118,11 +118,11 @@ class UnionTypeMask implements TypeMask {
         // TODO(sigmund, johnniwinther): computing length here (and below) is
         // expensive. If we can't prevent `flatten` from being called a lot, it
         // might be worth caching results.
-        size = classWorld.strictSubclassesOf(candidate).length;
-        assert(size <= classWorld.strictSubtypesOf(candidate).length);
+        size = classWorld.strictSubclassCount(candidate);
+        assert(size <= classWorld.strictSubtypeCount(candidate));
       } else {
         kind = FlatTypeMask.SUBTYPE;
-        size = classWorld.strictSubtypesOf(candidate).length;
+        size = classWorld.strictSubtypeCount(candidate);
       }
       // Update the best candidate if the new one is better.
       if (bestElement == null || size < bestSize) {