Add missing widget of the week video links to API docs (#76251)

packages/flutter/lib/src/material/flexible_space_bar.dart

@@ -38,6 +38,8 @@ enum StretchMode {
 /// The part of a material design [AppBar] that expands, collapses, and
 /// stretches.
 ///
+/// {@youtube 560 315 https://www.youtube.com/watch?v=mSc7qFzxHDw}
+///
 /// Most commonly used in the [SliverAppBar.flexibleSpace] field, a flexible
 /// space bar expands and contracts as the app scrolls so that the [AppBar]
 /// reaches from the top of the app to the top of the scrolling contents of the

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

@@ -908,6 +908,8 @@ class ClipPath extends SingleChildRenderObjectWidget {
 
 /// A widget representing a physical layer that clips its children to a shape.
 ///
+/// {@youtube 560 315 https://www.youtube.com/watch?v=XgUOSS30OQk}
+///
 /// Physical layers cast shadows based on an [elevation] which is nominally in
 /// logical pixels, coming vertically out of the rendering surface.
 ///

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

@@ -11,6 +11,8 @@ import 'framework.dart';
 
 /// Applies an [ImageFilter] to its child.
 ///
+/// {@youtube 560 315 https://www.youtube.com/watch?v=7Lftorq4i2o}
+///
 /// See also:
 ///
 /// * [BackdropFilter], which applies an [ImageFilter] to everything