reverted:
--- b/core/modules/migrate/tests/src/Kernel/MigrateSourceTestBase.php
+++ a/core/modules/migrate/tests/src/Kernel/MigrateSourceTestBase.php
@@ -158,8 +158,8 @@
       $expected_count = count($expected_data);
     }
     // If an expected count was given, assert it only if the plugin is
+    // countable.
+    if (is_numeric($expected_count)) {
-    // countable and there is no high water mark.
-    if (is_numeric($expected_count) && !isset($high_water)) {
       $this->assertInstanceOf('\Iterator', $plugin);
       $this->assertSame($expected_count, iterator_count($plugin));
     }
@@ -185,10 +185,6 @@
         }
       }
     }
-
-    if (isset($high_water)) {
-      $this->assertSame($expected_count, $i);
-    }
   }
 
 }