Don't crash attempting to JSON print an uninitialized Map.

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index 0765fcc28fc31f9c6a14df52823b0377d89249e8..4e3919df9ae6b8878231ac8a183e1594b55fc1a0 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -8200,6 +8200,10 @@ class LinkedHashMap : public Instance {
   }
 
   intptr_t Length() const {
+    // The map may be uninitialized.
+    if (raw_ptr()->used_data_ == Object::null()) return 0;
+    if (raw_ptr()->deleted_keys_ == Object::null()) return 0;
+
     intptr_t used = Smi::Value(raw_ptr()->used_data_);
     intptr_t deleted = Smi::Value(raw_ptr()->deleted_keys_);
     return (used >> 1) - deleted;