Issue 5162. Report error when initializing already initialized final field

samples/ui_lib/observable/ChangeEvent.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.
 
 /** A change to an observable instance. */
 class ChangeEvent {
   // TODO(sigmund): capture language issues around enums & create a cannonical
   // Dart enum design.
   /** Type denoting an in-place update event. */
   static const UPDATE = 0;
@@ skipping 10 matching lines @@
    */
   static const GLOBAL = 3;
 
   /** The observable instance that changed. */
   final Observable target;
 
   /** Whether the change was an [INSERT], [REMOVE], or [UPDATE]. */
   final int type;
 
   /** The value after the change (or inserted value in a list). */
-  final newValue = null;
+  final newValue;
 
   /** The value before the change (or removed value from a list). */
-  final oldValue = null;
+  final oldValue;
 
   /** Property that changed (null for list changes). */
-  final String propertyName = null;
+  final String propertyName;
 
   /**
    * Index of the list operation. Insertions prepend in front of the given
    * index (insert at 0 means an insertion at the beginning of the list).
    */
-  final int index = null;
+  final int index;
 
   /** Factory constructor for property change events. */
   ChangeEvent.property(
       this.target, this.propertyName, this.newValue, this.oldValue)
-    : type = UPDATE;
+    : type = UPDATE, index = null;
 
   /** Factory constructor for list change events. */
   ChangeEvent.list(
-      this.target, this.type, this.index, this.newValue, this.oldValue);
+      this.target, this.type, this.index, this.newValue, this.oldValue)
+    : propertyName = null;
 
   /** Factory constructor for [GLOBAL] change events. */
-  ChangeEvent.global(this.target) : type = GLOBAL;
+  ChangeEvent.global(this.target)
+    : type = GLOBAL, newValue = null, oldValue = null, propertyName = null, index = null;
 }
 
 /** A collection of change events on a single observable instance. */
 class EventSummary {
   final Observable target;
 
   // TODO(sigmund): evolve this to track changes per property.
   List<ChangeEvent> events;
 
   EventSummary(this.target) : events = new List<ChangeEvent>();
 
   void addEvent(ChangeEvent e) {
     events.add(e);
   }
 
   /** Notify listeners of [target] and parents of [target] about all changes. */
   void notify() {
     if (!events.isEmpty()) {
       for (Observable obj = target; obj != null; obj = obj.parent) {
         for (final listener in obj.listeners) {
           listener(this);
         }
       }
     }
   }
 }
 
 /** A listener of change events. */
 typedef void ChangeListener(EventSummary events);