Provide trace in single transformer (#1607)

drift/lib/src/utils/single_transformer.dart

@@ -4,16 +4,17 @@ import 'dart:async';
 /// that each list is a singleton or empty.
 StreamTransformer<List<T>, T?> singleElementsOrNull<T>() {
   return StreamTransformer.fromHandlers(handleData: (data, sink) {
+    T? result;
+
     try {
-      if (data.isEmpty) {
+      if (data.isNotEmpty) {
-        sink.add(null);
+        result = data.single;
-      } else {
-        sink.add(data.single);
       }
     } catch (e) {
-      sink.addError(
+      throw StateError('Expected exactly one element, but got ${data.length}');
-          StateError('Expected exactly one element, but got ${data.length}'));
     }
+
+    sink.add(result);
   });
 }
 
@@ -21,11 +22,14 @@ StreamTransformer<List<T>, T?> singleElementsOrNull<T>() {
 /// that each list is a singleton.
 StreamTransformer<List<T>, T> singleElements<T>() {
   return StreamTransformer.fromHandlers(handleData: (data, sink) {
+    T single;
+
     try {
-      sink.add(data.single);
+      single = data.single;
     } catch (e) {
-      sink.addError(
+      throw StateError('Expected exactly one element, but got ${data.length}');
-          StateError('Expected exactly one element, but got ${data.length}'));
     }
+
+    sink.add(single);
   });
 }

drift/test/utils/single_transformer_test.dart

@@ -22,8 +22,15 @@ void main() {
     final controller = StreamController<List<int>>();
     final stream = controller.stream.transform(singleElements());
 
-    expectLater(stream,
+    expectLater(
-        emitsInOrder([1, emitsError(anything), 2, emitsError(anything)]));
+      stream,
+      emitsInOrder([
+        1,
+        emitsError(_stateErrorWithTrace),
+        2,
+        emitsError(_stateErrorWithTrace)
+      ]),
+    );
 
     controller
       ..add([1])
@@ -38,3 +45,6 @@ void main() {
     expect(stream.transform(singleElementsOrNull()), emits(isNull));
   });
 }
+
+Matcher _stateErrorWithTrace =
+    isStateError.having((e) => e.stackTrace, 'stackTrace', isNotNull);