When clearing a field make the field state "does not exist"

lib/generated_message.dart

 // Copyright (c) 2011, 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.
 
 part of protobuf;
 
 typedef GeneratedMessage CreateBuilderFunc();
 typedef Object MakeDefaultFunc();
 typedef ProtobufEnum ValueOfFunc(int value);
 
@@ skipping 856 matching lines @@
     var value = extension.makeDefault();
     if (value is List) {
       _setExtension(extension, value);
     } else {
       _extensions.remove(extension.tagNumber);
     }
   }
 
   void clearField(int tagNumber) {
     _fieldValues.remove(tagNumber);
-    // Restore the default value for the field.
-    // For repeated fields, the default is an empty list.
-    // For required/optional fields, the BuilderInfo contains the initializer.
-    int fieldType = _getFieldType(tagNumber);
-    if ((fieldType & _REPEATED_BIT) != 0) {
-      _fieldValues[tagNumber] = new PbList();
-    } else {
-      var defaultValue = info_.defaultValue(tagNumber);
-      if (defaultValue != null) {
-        _fieldValues[tagNumber] = defaultValue;
-      }
-    }
   }
 
   bool extensionsAreInitialized() {
     return _extensions.keys.every((int tagNumber) {
       return info_._isFieldInitialized(_fieldValues, tagNumber,
           _extensions[tagNumber].type);
     });
   }
 
   /**
@@ skipping 337 matching lines @@
               _generateMessage(tagNumber, value, 'not a GeneratedMessage'));
         }
         break;
       default:
         throw new ArgumentError(
             _generateMessage(tagNumber, value, 'field has unknown type '
             '$fieldType'));
     }
   }
 }