Update BlendMode #4

- Update from Flutter.

lib/src/api/models/blend_mode.dart

@@ -163,6 +163,12 @@ enum BlendModeC {
   ///
   /// This corresponds to the "Source plus Destination" Porter-Duff operator.
   ///
+  /// This is the right blend mode for cross-fading between two images. Consider
+  /// two images A and B, and an interpolation time variable _t_ (from 0.0 to
+  /// 1.0). To cross fade between them, A should be drawn with opacity 1.0 - _t_
+  /// into a new layer using [BlendMode.srcOver], and B should be drawn on top
+  /// of it, at opacity _t_, into the same layer, using [BlendMode.plus].
+  ///
   /// ![](https://flutter.github.io/assets-for-api-docs/assets/dart-ui/blend_mode_plus.png)
   plus,
 
@@ -380,7 +386,7 @@ enum BlendModeC {
   /// [srcOver]. Regions that are entirely transparent in the source image take
   /// their saturation from the destination.
   ///
-  /// ![](https://flutter.github.io/assets-for-api-docs/assets/dart-ui/blend_mode_hue.png)
+  /// ![](https://flutter.github.io/assets-for-api-docs/assets/dart-ui/blend_mode_saturation.png)
   ///
   /// See also:
   ///