Reapply change to hide VM-only List implementation classes.

runtime/vm/intrinsifier_x64.cc

Index: runtime/vm/intrinsifier_x64.cc
diff --git a/runtime/vm/intrinsifier_x64.cc b/runtime/vm/intrinsifier_x64.cc
index 79dba1e40340f3a3ab4a4f654d5258e3b272f180..2b812f90d1f244dd0d3d7ea75c4d6c2061767315 100644
--- a/runtime/vm/intrinsifier_x64.cc
+++ b/runtime/vm/intrinsifier_x64.cc
@@ -1237,15 +1237,17 @@ static intptr_t GetOffsetForField(const char* class_name_p,
                                   const char* field_name_p) {
   const String& class_name = String::Handle(Symbols::New(class_name_p));
   const String& field_name = String::Handle(Symbols::New(field_name_p));
-  const Class& cls = Class::Handle(Library::Handle(
-      Library::CoreImplLibrary()).LookupClass(class_name));
+  const Library& coreimpl_lib = Library::Handle(Library::CoreImplLibrary());
+  const Class& cls =
+      Class::Handle(coreimpl_lib.LookupClassAllowPrivate(class_name));
   ASSERT(!cls.IsNull());
   const Field& field = Field::ZoneHandle(cls.LookupInstanceField(field_name));
   ASSERT(!field.IsNull());
   return field.Offset();
 }
 
-static const char* kFixedSizeArrayIteratorClassName = "FixedSizeArrayIterator";
+
+static const char* kFixedSizeArrayIteratorClassName = "_FixedSizeArrayIterator";
 
 // Class 'FixedSizeArrayIterator':
 //   T next() {