Test Iterable.flat propagates .is-lazy

Note that routine &flat does not propagate it and based on the
source comments[^1], it appears it wasn't meant to.

[1] https://github.com/rakudo/rakudo/blob/51b0aba/src/core/List.pm#L1377-L1382

Author: zoffixznet

S02-types/array.t

@@ -2,7 +2,7 @@ use v6;
 
 use Test;
 
-plan 102;
+plan 103;
 
 #L<S02/Mutable types/Array>
 
@@ -391,4 +391,11 @@ my @array2 = ("test", 1, Mu);
     cmp-ok $x.flat,  '===', $x, 'Array:U.flat is identity';
 }
 
+# https://github.com/rakudo/rakudo/commit/51b0aba8e8
+subtest '.flat propagates .is-lazy' => {
+    plan 2;
+    is-deeply (42 xx *).flat.is-lazy, True,  'method, True';
+    is-deeply (42 xx 1).flat.is-lazy, False, 'method, False';
+}
+
 # vim: ft=perl6