Use reportError (over onError) everywhere (#63784)

packages/flutter/lib/src/foundation/assertions.dart

@@ -871,6 +871,9 @@ class FlutterError extends Error with DiagnosticableTreeMixin implements Asserti
   ///
   /// Set this to null to silently catch and ignore errors. This is not
   /// recommended.
+  ///
+  /// Do not call [onError] directly, instead, call [reportError], which
+  /// forwards to [onError] if it is not null.
   static FlutterExceptionHandler? onError = (FlutterErrorDetails details) => presentError(details);
 
   /// Called by the Flutter framework before attempting to parse a [StackTrace].

packages/flutter/lib/src/painting/image_provider.dart

@@ -401,14 +401,12 @@ abstract class ImageProvider<T extends Object> {
             };
             return true;
           }());
-          if (FlutterError.onError != null) {
-            FlutterError.onError!(FlutterErrorDetails(
+          FlutterError.reportError(FlutterErrorDetails(
             context: ErrorDescription('while checking the cache location of an image'),
             informationCollector: collector,
             exception: exception,
             stack: stack,
           ));
-          }
           completer.complete(null);
         }
       },