Remove Dart_VisitPrologueWeakHandles from the API header.

runtime/include/dart_api.h

Index: runtime/include/dart_api.h
diff --git a/runtime/include/dart_api.h b/runtime/include/dart_api.h
index 42ba0f2491e3c4ba0b0744bcbc026decfa2e8208..9ea762dc772b95b80135c1c5ee28e332acb1d0ab 100755
--- a/runtime/include/dart_api.h
+++ b/runtime/include/dart_api.h
@@ -586,13 +586,6 @@ DART_EXPORT Dart_Handle Dart_SetGcCallbacks(
     Dart_GcPrologueCallback prologue_callback,
     Dart_GcEpilogueCallback epilogue_callback);
 
-typedef void (*Dart_GcPrologueWeakHandleCallback)(void* isolate_callback_data,
-                                                  Dart_WeakPersistentHandle obj,
-                                                  intptr_t num_native_fields,
-                                                  intptr_t* native_fields);
-
-DART_EXPORT Dart_Handle Dart_VisitPrologueWeakHandles(
-    Dart_GcPrologueWeakHandleCallback callback);
 
 /*
  * ==========================