As of now reason is a positional argument of expect.

tests/html/canvas_pixel_array_type_alias_test.dart

Index: tests/html/canvas_pixel_array_type_alias_test.dart
diff --git a/tests/html/canvas_pixel_array_type_alias_test.dart b/tests/html/canvas_pixel_array_type_alias_test.dart
index 8ef7565fcb168b8bdf20e46be953f96a8a023de0..5560d9f96e9932a2fac457dd636e8866c4028fab 100644
--- a/tests/html/canvas_pixel_array_type_alias_test.dart
+++ b/tests/html/canvas_pixel_array_type_alias_test.dart
@@ -31,7 +31,6 @@ main() {
     Uint8ClampedArray data = image.data;
     // It is legal for the dart2js compiler to believe the type of the native
     // ImageData.data and elides the check, so check the type explicitly:
-    // OPTIONALS expect(confuseType(data) is Uint8ClampedArray, reason: 'canvas array type');
     expect(confuseType(data) is Uint8ClampedArray, isTrue, 'canvas array type');
 
     expect(data, hasLength(40000));