fix(dccd): fix false positive for collection moves

Closes dart-archive#1105

Author: vicb

lib/change_detection/dirty_checking_change_detector.dart

@@ -1110,7 +1110,7 @@ class _CollectionChangeRecord<V> implements CollectionChangeRecord<V> {
       record = _reinsertAfter(reinsertRecord, record._prev, index);
     } else if (record.currentIndex != index) {
       record.currentIndex = index;
-      _addToMoves(record);
+      _addToMoves(record, index);
     }
     return record;
   }
@@ -1152,14 +1152,14 @@ class _CollectionChangeRecord<V> implements CollectionChangeRecord<V> {
     }
 
     _insertAfter(record, prevRecord, index);
-    _addToMoves(record);
+    _addToMoves(record, index);
     return record;
   }
 
   ItemRecord<V> _moveAfter(ItemRecord<V> record, ItemRecord<V> prevRecord, int index) {
     _unlink(record);
     _insertAfter(record, prevRecord, index);
-    _addToMoves(record);
+    _addToMoves(record, index);
     return record;
   }
 
@@ -1228,8 +1228,11 @@ class _CollectionChangeRecord<V> implements CollectionChangeRecord<V> {
     return record;
   }
 
-  ItemRecord<V> _addToMoves(ItemRecord<V> record) {
+  ItemRecord<V> _addToMoves(ItemRecord<V> record, int toIndex) {
     assert(record._nextMoved == null);
+
+    if (record.previousIndex == toIndex) return record;
+
     if (_movesTail == null) {
       assert(_movesHead == null);
       _movesTail = _movesHead = record;

test/change_detection/dirty_checking_change_detector_spec.dart

@@ -301,7 +301,7 @@ void testWithGetterFactory(FieldGetterFactory getterFactory) {
               collection: ['b[1 -> 0]', 'a[0 -> 1]', 'c'],
               previous: ['a[0 -> 1]', 'b[1 -> 0]', 'c'],
               additions: [],
-              moves: ['b[1 -> 0]', 'a[0 -> 1]', 'c'],
+              moves: ['b[1 -> 0]', 'a[0 -> 1]'],
               removals: []));
 
           list..clear()..addAll(['b', 'c', 'a']);
@@ -532,6 +532,21 @@ void testWithGetterFactory(FieldGetterFactory getterFactory) {
             moves: ['(1)a-a[0 -> 1]'],
             removals: []));
       });
+
+      it('should not report unnecessary moves', () {
+        var list = ['a', 'b', 'c'];
+        var record = detector.watch(list, null, null);
+        var iterator = detector.collectChanges()..moveNext();
+
+        list..clear()..addAll(['b', 'a', 'c']);
+        iterator = detector.collectChanges()..moveNext();
+        expect(iterator.current.currentValue, toEqualCollectionRecord(
+            collection: ['b[1 -> 0]', 'a[0 -> 1]', 'c'],
+            previous: ['a[0 -> 1]', 'b[1 -> 0]', 'c'],
+            additions: [],
+            moves: ['b[1 -> 0]', 'a[0 -> 1]'],
+            removals: []));
+      });
     });
 
     describe('map watching', () {