Avoid scanning huge freelists for large enough free blocks.

runtime/vm/freelist.h

Index: runtime/vm/freelist.h
diff --git a/runtime/vm/freelist.h b/runtime/vm/freelist.h
index 068c4e21e84514e2dc1aee6aaa6a55cd8fb93e6d..f8a8e82f66ad863facacfbaed7d53db209dd7191 100644
--- a/runtime/vm/freelist.h
+++ b/runtime/vm/freelist.h
@@ -98,6 +98,7 @@ class FreeList {
 
  private:
   static const int kNumLists = 128;
+  static const intptr_t kInitialFreeListSearchBudget = 1000;
 
   static intptr_t IndexForSize(intptr_t size);
 
@@ -120,6 +121,8 @@ class FreeList {
 
   FreeListElement* free_lists_[kNumLists + 1];
 
+  intptr_t freelist_search_budget_;
+
   // The largest available small size in bytes, or negative if there is none.
   intptr_t last_free_small_size_;