Fix GCC compiler warning

runtime/vm/dart_api_message.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/dart_api_message.h"
 #include "vm/object.h"
 #include "vm/snapshot_ids.h"
 #include "vm/symbols.h"
 #include "vm/unicode.h"
 
@@ skipping 1006 matching lines @@
       // Write out the class and tags information.
       intptr_t class_id;
       switch (object->value.as_typed_data.type) {
         case Dart_CObject::kInt8Array:
           class_id = kTypedDataInt8ArrayCid;
           break;
         case Dart_CObject::kUint8Array:
           class_id = kTypedDataUint8ArrayCid;
           break;
         default:
+          class_id = kTypedDataUint8ArrayCid;
           UNIMPLEMENTED();
       }
 
       WriteIndexedObject(class_id);
       WriteIntptrValue(RawObject::ClassIdTag::update(class_id, 0));
       uint8_t* bytes = object->value.as_typed_data.values;
       intptr_t len = object->value.as_typed_data.length;
       WriteSmi(len);
       for (intptr_t i = 0; i < len; i++) {
         Write<uint8_t>(bytes[i]);
@@ skipping 45 matching lines @@
     if (!success) {
       UnmarkAllCObjects(object);
       return false;
     }
   }
   UnmarkAllCObjects(object);
   return true;
 }
 
 }  // namespace dart