Make sure that ICData always contains valid data (non-null values).

runtime/vm/object.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_OBJECT_H_
 #define VM_OBJECT_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "platform/utils.h"
@@ skipping 3304 matching lines @@
   // num_args_tested == 1.
   void AddReceiverCheck(intptr_t receiver_class_id,
                         const Function& target,
                         intptr_t count = 1) const;
 
   // Retrieving checks.
 
   void GetCheckAt(intptr_t index,
                   GrowableArray<intptr_t>* class_ids,
                   Function* target) const;
+  // Only for 'num_args_checked == 1'.
   void GetOneClassCheckAt(intptr_t index,
                           intptr_t* class_id,
                           Function* target) const;
   intptr_t GetReceiverClassIdAt(intptr_t index) const;
   intptr_t GetClassIdAt(intptr_t index, intptr_t arg_nr) const;
 
   RawFunction* GetTargetAt(intptr_t index) const;
   RawFunction* GetTargetForReceiverClassId(intptr_t class_id) const;
 
   void IncrementCountAt(intptr_t index, intptr_t value) const;
@@ skipping 2695 matching lines @@
 
 
 RawObject* MegamorphicCache::GetTargetFunction(const Array& array,
                                                intptr_t index) {
   return array.At((index * kEntryLength) + kTargetFunctionIndex);
 }
 
 }  // namespace dart
 
 #endif  // VM_OBJECT_H_