Add assert for mutually exclusive errorBuilder and errorText [Flutter core]

packages/dropdown_button2/CHANGELOG.md

@@ -5,6 +5,7 @@
 - Add `forceErrorText` support for DropdownButtonFormField2 [Flutter core].
 - Add ARIA menu roles to menu-related widgets for accessibility [Flutter core].
 - Document `DropdownButtonFormField2.onChanged` ordering relative to `Form.onChanged` [Flutter core].
+- Add assert for mutually exclusive `errorBuilder` and `errorText` [Flutter core].
 - Add `anchoredMinHeight` to keep the menu anchored to the button and shrink to fit when possible, closes #429.
 - Re-lay out dropdown menu on ancestor scroll.
 - Add `barrierBlocksInteraction` to allow interaction with underlying widgets while the dropdown menu is open.

packages/dropdown_button2/lib/src/dropdown_button2.dart

@@ -1153,6 +1153,10 @@ class DropdownButtonFormField2<T> extends FormField<T> {
          valueListenable == null || multiValueListenable == null,
          'Only one of valueListenable or multiValueListenable can be used.',
        ),
+       assert(
+         errorBuilder == null || decoration?.errorText == null,
+         'Declaring both errorBuilder and decoration.errorText is not supported.',
+       ),
        decoration = decoration ?? const InputDecoration(),
        super(
          initialValue: valueListenable != null