fix inconsistent firing of events for extension fields

lib/src/protobuf/event_plugin.dart

 part of protobuf;
 
 /// An EventPlugin receives callbacks when the fields of a GeneratedMessage
 /// change.
 ///
 /// A GeneratedMessage mixin can install a plugin by overriding the eventPlugin
 /// property. The intent is provide mechanism, not policy; each mixin defines
 /// its own public API, perhaps using streams.
 ///
 /// This is a low-level, synchronous API. Event handlers are called in the
 /// middle of protobuf changes. To avoid exposing half-finished changes
 /// to user code, plugins should buffer events and send them asynchronously.
 /// (See event_mixin.dart for an example.)
 abstract class EventPlugin {
 
   /// Initializes the plugin.
   ///
   /// GeneratedMessage calls this once in its constructors.
   void attach(GeneratedMessage parent);
 
   /// If false, GeneratedMessage will skip calls to event handlers.
   bool get hasObservers;
 
-  /// Called before setting a field if startGroup returned true.
+  /// Called before setting a non-extension field.
   ///
   /// For repeated fields, this will be called in getField() when
   /// the PbList is created.
   void beforeSetField(int tag, newValue);
 
-  /// Called before clearing a field if startGroup returned true.
+  /// Called before clearing a non-extension field.
   void beforeClearField(int tag);
 }