Make onImage re-entrant safe (#50316)

packages/flutter/lib/src/widgets/image.dart

@@ -88,11 +88,15 @@ Future<void> precacheImage(
   ImageStreamListener listener;
   listener = ImageStreamListener(
     (ImageInfo image, bool sync) {
-      completer.complete();
+      if (!completer.isCompleted) {
+        completer.complete();
+      }
       stream.removeListener(listener);
     },
     onError: (dynamic exception, StackTrace stackTrace) {
-      completer.complete();
+      if (!completer.isCompleted) {
+        completer.complete();
+      }
       stream.removeListener(listener);
       if (onError != null) {
         onError(exception, stackTrace);

packages/flutter/test/widgets/image_test.dart

@@ -639,13 +639,17 @@ void main() {
       )
     );
 
-    expect(imageStreamCompleter.listeners.length, 2);
-    imageStreamCompleter.listeners.toList()[1].onImage(null, null);
+    // Two listeners - one is the listener added by precacheImage, the other by the ImageCache.
+    final List<ImageStreamListener> listeners = imageStreamCompleter.listeners.toList();
+    expect(listeners.length, 2);
 
-    expect(imageStreamCompleter.listeners.length, 1);
-    imageStreamCompleter.listeners.toList()[0].onImage(null, null);
+    // Make sure the first listener can be called re-entrantly
+    listeners[1].onImage(null, null);
+    listeners[1].onImage(null, null);
 
-    expect(imageStreamCompleter.listeners.length, 0);
+    // Make sure the second listener can be called re-entrantly.
+    listeners[0].onImage(null, null);
+    listeners[0].onImage(null, null);
   });
 
   testWidgets('Precache completes with onError on error', (WidgetTester tester) async {