Ensure that BoxInt32 input is correctly preserved on the slow-path.

runtime/vm/locations.h

Index: runtime/vm/locations.h
diff --git a/runtime/vm/locations.h b/runtime/vm/locations.h
index a5332391e9337652b17300c433e6905da2aa90b1..5438b0868a2095f69624ceb908a1016bd9377e5d 100644
--- a/runtime/vm/locations.h
+++ b/runtime/vm/locations.h
@@ -631,6 +631,13 @@ class LocationSummary : public ZoneAllocated {
     return &live_registers_;
   }
 
+#if defined(DEBUG)
+  // Debug only verification that ensures that writable registers are correctly
+  // preserved on the slow path.
+  void DiscoverWritableInputs();
+  void CheckWritableInputs();
+#endif
+
  private:
   const intptr_t num_inputs_;
   Location* input_locations_;
@@ -642,6 +649,10 @@ class LocationSummary : public ZoneAllocated {
 
   const ContainsCall contains_call_;
   RegisterSet live_registers_;
+
+#if defined(DEBUG)
+  intptr_t writable_inputs_;
+#endif
 };