Fix to ListMultimapBuilder.[].

Author: davidmorgan

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changelog
 
+## 4.2.2 (unreleased)
+
+- Bug fix: `ListMultimapBuilder.[]` no longer ignores modifications under some
+  circumstances.
+
 ## 4.2.1
 
 - Bug fix: `ListBuilder` `first` and `last` setters can no longer modify the

lib/src/list_multimap/list_multimap_builder.dart

@@ -166,8 +166,10 @@ class ListMultimapBuilder<K, V> {
   }
 
   /// As [ListMultimap], but results are [ListBuilder]s.
-  ListBuilder<V> operator [](Object key) =>
-      key is K ? _getValuesBuilder(key) : new ListBuilder<V>();
+  ListBuilder<V> operator [](Object key) {
+    _makeWriteableCopy();
+    return key is K ? _getValuesBuilder(key) : new ListBuilder<V>();
+  }
 
   // Internal.
 

test/list_multimap/list_multimap_builder_test.dart

@@ -421,7 +421,8 @@ void main() {
           {});
     });
 
-    test('has a method like ListMultimap[]', () {
+    test('has a method like ListMultimap[] which can be used to read values',
+        () {
       expect(
           new BuiltListMultimap<int, String>({
             1: ['1'],
@@ -437,5 +438,12 @@ void main() {
           }).toBuilder()[4].build().toList(),
           []);
     });
+
+    test('has a method like ListMultimap[] which can be used to write values',
+        () {
+      var builder = BuiltListMultimap<int, String>().toBuilder();
+      builder[1].add('1');
+      expect(builder.build()[1], ['1']);
+    });
   });
 }