Never try to add 0 bytes to free list.

runtime/vm/pages.cc

Index: runtime/vm/pages.cc
diff --git a/runtime/vm/pages.cc b/runtime/vm/pages.cc
index 503d2b2d6a2be765ba4c897e2ebcc63475d83eaa..0aca3477a69152c206791224f90ddbe05b1ba7c3 100644
--- a/runtime/vm/pages.cc
+++ b/runtime/vm/pages.cc
@@ -220,7 +220,10 @@ uword PageSpace::TryAllocate(intptr_t size,
       result = page->object_start();
       // Enqueue the remainder in the free list.
       uword free_start = result + size;
-      freelist_[type].Free(free_start, page->object_end() - free_start);
+      intptr_t free_size = page->object_end() - free_start;
+      if (free_size > 0) {
+        freelist_[type].Free(free_start, free_size);
+      }
     }
   } else {
     // Large page allocation.