Fix json message in debugger wire protocol

runtime/platform/json.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 "platform/json.h"
 
 #include "platform/assert.h"
 #include "platform/globals.h"
 #include "platform/utils.h"
 #include "vm/os.h"
@@ skipping 237 matching lines @@
     case JSONScanner::TokenNull:
       scanner_.Scan();
       break;
     default:
       OS::Print("Malformed JSON: expected a value but got '%s'\n",
                 scanner_.TokenChars());
       FATAL("illegal JSON value found");
   }
 }
 
+
+#if defined (DEBUG)
 #define CHECK_TOKEN(token)                                                     \
   if (scanner_.CurrentToken() != token) {                                      \
     OS::Print("Malformed JSON: expected %s but got '%s'\n",                    \
               #token, scanner_.TokenChars());                                  \
-  }                                                                            \
-  ASSERT(scanner_.CurrentToken() == token);
+    intptr_t offset = scanner_.TokenChars() - this->json_object_;              \
+    OS::Print("Malformed JSON: expected %s at offset %" Pd "of buffer:\n%s\n", \
+              #token, offset, this->json_object_);                             \
+    ASSERT(scanner_.CurrentToken() == token);                                  \
+  }
+#else
+#define CHECK_TOKEN(token)
+#endif
+
 
 void JSONReader::CheckArray() {
   CHECK_TOKEN(JSONScanner::TokenLBrack);
   scanner_.Scan();
   while (scanner_.CurrentToken() != JSONScanner::TokenRBrack) {
     CheckValue();
     if (scanner_.CurrentToken() != JSONScanner::TokenComma) {
       break;
     }
     scanner_.Scan();
@@ skipping 360 matching lines @@
     // the debugger front-end.
     intptr_t new_size = buf_size_ + len + kBufferSpareCapacity;
     char* new_buf = reinterpret_cast<char*>(realloc(buf_, new_size));
     ASSERT(new_buf != NULL);
     buf_ = new_buf;
     buf_size_ = new_size;
   }
 }
 
 }  // namespace dart