Fix one missing invalid encoding case.

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 16744 matching lines @@
         }
         int32_t code_point = cpi.Current();
         if (!IsHexCharacter(code_point)) {
           return String::null();
         }
         if (!cpi.Next()) {
           return String::null();
         }
         code_point = cpi.Current();
         if (!IsHexCharacter(code_point)) {
-          return str.raw();
+          return String::null();
         }
         num_escapes += 2;
       }
     }
   }
   intptr_t utf8_len = len - num_escapes;
   ASSERT(utf8_len >= 0);
   Zone* zone = Isolate::Current()->current_zone();
   uint8_t* utf8 = zone->Alloc<uint8_t>(utf8_len);
   {
@@ skipping 2689 matching lines @@
   return tag_label.ToCString();
 }
 
 
 void UserTag::PrintJSONImpl(JSONStream* stream, bool ref) const {
   Instance::PrintJSONImpl(stream, ref);
 }
 
 
 }  // namespace dart