Fix flaky test failure:

runtime/vm/object.cc

 // 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.
 
 #include "vm/object.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/cpu.h"
@@ skipping 15228 matching lines @@
 #endif  // DEBUG
 
 
 bool Instance::CheckAndCanonicalizeFields(Thread* thread,
                                           const char** error_str) const {
   if (GetClassId() >= kNumPredefinedCids) {
     // Iterate over all fields, canonicalize numbers and strings, expect all
     // other instances to be canonical otherwise report error (return false).
     Zone* zone = thread->zone();
     Object& obj = Object::Handle(zone);
-    intptr_t end_field_offset = SizeFromClass() - kWordSize;
-    for (intptr_t field_offset = 0;
-         field_offset <= end_field_offset;
-         field_offset += kWordSize) {
-      obj = *this->FieldAddrAtOffset(field_offset);
+    const intptr_t instance_size = SizeFromClass();
+    ASSERT(instance_size != 0);
+    for (intptr_t offset = Instance::NextFieldOffset();
+         offset < instance_size;
+         offset += kWordSize) {
+      obj = *this->FieldAddrAtOffset(offset);
       if (obj.IsInstance() && !obj.IsSmi() && !obj.IsCanonical()) {
         if (obj.IsNumber() || obj.IsString()) {
           obj = Instance::Cast(obj).CheckAndCanonicalize(thread, NULL);
           ASSERT(!obj.IsNull());
-          this->SetFieldAtOffset(field_offset, obj);
+          this->SetFieldAtOffset(offset, obj);
         } else {
           ASSERT(error_str != NULL);
           char* chars = OS::SCreate(zone, "field: %s\n", obj.ToCString());
           *error_str = chars;
           return false;
         }
       }
     }
   } else {
 #if defined(DEBUG)
@@ skipping 7340 matching lines @@
   return UserTag::null();
 }
 
 
 const char* UserTag::ToCString() const {
   const String& tag_label = String::Handle(label());
   return tag_label.ToCString();
 }
 
 }  // namespace dart