Revert "Remove @deprecated features."

pkg/observe/lib/src/observable_map.dart

Index: pkg/observe/lib/src/observable_map.dart
diff --git a/pkg/observe/lib/src/observable_map.dart b/pkg/observe/lib/src/observable_map.dart
index 1223ca335f1bfed90ba762d64244cf11b47c8640..d4da6461725ed7fa29f5f4b7293f1a033523a232 100644
--- a/pkg/observe/lib/src/observable_map.dart
+++ b/pkg/observe/lib/src/observable_map.dart
@@ -39,6 +39,10 @@ class MapChangeRecord<K, V> extends ChangeRecord {
   MapChangeRecord.remove(this.key, this.oldValue)
       : isInsert = false, isRemove = true;
 
+  /// *Deprecated* compare [key]s instead.
+  @deprecated
+  bool changes(otherKey) => key == otherKey;
+
   String toString() {
     var kind = isInsert ? 'insert' : isRemove ? 'remove' : 'set';
     return '#<MapChangeRecord $kind $key from: $oldValue to: $newValue>';