Add links to implicit animation illustrations. (#21092)

This adds links to the already existing animated diagrams in the asset-for-api-docs repo.

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

@@ -191,6 +191,10 @@ class ThemeDataTween extends Tween<ThemeData> {
 /// Animated version of [Theme] which automatically transitions the colors,
 /// etc, over a given duration whenever the given theme changes.
 ///
+/// Here's an illustration of what using this widget looks like, using a [curve]
+/// of [Curves.elasticInOut].
+/// {@animation 250 266 https://flutter.github.io/assets-for-api-docs/assets/widgets/animated_theme.mp4}
+///
 /// See also:
 ///
 ///  * [Theme], which [AnimatedTheme] uses to actually apply the interpolated

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

@@ -404,11 +404,20 @@ abstract class AnimatedWidgetBaseState<T extends ImplicitlyAnimatedWidget> exten
 /// [AnimatedWidget] such as the [DecoratedBoxTransition] or use your own
 /// [AnimationController].
 ///
+/// Here's an illustration of what using this widget looks like, using a [curve]
+/// of [Curves.fastOutSlowIn].
+/// {@animation 250 266 https://flutter.github.io/assets-for-api-docs/assets/widgets/animated_container.mp4}
+///
 /// See also:
 ///
 ///  * [AnimatedPadding], which is a subset of this widget that only
 ///    supports animating the [padding].
 ///  * The [catalog of layout widgets](https://flutter.io/widgets/layout/).
+///  * [AnimatedPositioned], which, as a child of a [Stack], automatically
+///    transitions its child's position over a given duration whenever the given
+///    position changes.
+///  * [AnimatedAlign], which automatically transitions its child's
+///    position over a given duration whenever the given [alignment] changes.
 class AnimatedContainer extends ImplicitlyAnimatedWidget {
   /// Creates a container that animates its parameters implicitly.
   ///
@@ -564,9 +573,15 @@ class _AnimatedContainerState extends AnimatedWidgetBaseState<AnimatedContainer>
 /// Animated version of [Padding] which automatically transitions the
 /// indentation over a given duration whenever the given inset changes.
 ///
+/// Here's an illustration of what using this widget looks like, using a [curve]
+/// of [Curves.fastOutSlowIn].
+/// {@animation 250 266 https://flutter.github.io/assets-for-api-docs/assets/widgets/animated_padding.mp4}
+///
 /// See also:
 ///
 ///  * [AnimatedContainer], which can transition more values at once.
+///  * [AnimatedAlign], which automatically transitions its child's
+///    position over a given duration whenever the given [alignment] changes.
 class AnimatedPadding extends ImplicitlyAnimatedWidget {
   /// Creates a widget that insets its child by a value that animates
   /// implicitly.
@@ -626,9 +641,18 @@ class _AnimatedPaddingState extends AnimatedWidgetBaseState<AnimatedPadding> {
 /// Animated version of [Align] which automatically transitions the child's
 /// position over a given duration whenever the given [alignment] changes.
 ///
+/// Here's an illustration of what this can look like, using a [curve] of
+/// [Curves.fastOutSlowIn].
+/// {@animation 250 266 https://flutter.github.io/assets-for-api-docs/assets/widgets/animated_align.mp4}
+///
 /// See also:
 ///
 ///  * [AnimatedContainer], which can transition more values at once.
+///  * [AnimatedPadding], which can animate the padding instead of the
+///    alignment.
+///  * [AnimatedPositioned], which, as a child of a [Stack], automatically
+///    transitions its child's position over a given duration whenever the given
+///    position changes.
 class AnimatedAlign extends ImplicitlyAnimatedWidget {
   /// Creates a widget that positions its child by an alignment that animates
   /// implicitly.
@@ -711,6 +735,10 @@ class _AnimatedAlignState extends AnimatedWidgetBaseState<AnimatedAlign> {
 /// frame of the animation, whereas [AnimatedPositioned] will trigger a relayout
 /// as well.
 ///
+/// Here's an illustration of what using this widget looks like, using a [curve]
+/// of [Curves.fastOutSlowIn].
+/// {@animation 250 266 https://flutter.github.io/assets-for-api-docs/assets/widgets/animated_positioned.mp4}
+///
 /// See also:
 ///
 ///  * [AnimatedPositionedDirectional], which adapts to the ambient
@@ -860,6 +888,10 @@ class _AnimatedPositionedState extends AnimatedWidgetBaseState<AnimatedPositione
 /// frame of the animation, whereas [AnimatedPositionedDirectional] will trigger
 /// a relayout as well. ([SlideTransition] is also text-direction-aware.)
 ///
+/// Here's an illustration of what using this widget looks like, using a [curve]
+/// of [Curves.fastOutSlowIn].
+/// {@animation 250 266 https://flutter.github.io/assets-for-api-docs/assets/widgets/animated_positioned_directional.mp4}
+///
 /// See also:
 ///
 ///  * [AnimatedPositioned], which specifies the widget's position visually (the
@@ -982,6 +1014,10 @@ class _AnimatedPositionedDirectionalState extends AnimatedWidgetBaseState<Animat
 /// Animating an opacity is relatively expensive because it requires painting
 /// the child into an intermediate buffer.
 ///
+/// Here's an illustration of what using this widget looks like, using a [curve]
+/// of [Curves.fastOutSlowIn].
+/// {@animation 250 266 https://flutter.github.io/assets-for-api-docs/assets/widgets/animated_opacity.mp4}
+///
 /// ## Sample code
 ///
 /// ```dart
@@ -1016,6 +1052,11 @@ class _AnimatedPositionedDirectionalState extends AnimatedWidgetBaseState<Animat
 ///   }
 /// }
 /// ```
+///
+/// See also:
+///
+///  * [FadeTransition], an explicitly animated version of this widget, where
+///    an [Animation] is provided by the caller instead of being built in.
 class AnimatedOpacity extends ImplicitlyAnimatedWidget {
   /// Creates a widget that animates its opacity implicitly.
   ///
@@ -1083,6 +1124,10 @@ class _AnimatedOpacityState extends ImplicitlyAnimatedWidgetState<AnimatedOpacit
 ///
 /// The [textAlign], [softWrap], [textOverflow], and [maxLines] properties are
 /// not animated and take effect immediately when changed.
+///
+/// Here's an illustration of what using this widget looks like, using a [curve]
+/// of [Curves.elasticInOut].
+/// {@animation 250 266 https://flutter.github.io/assets-for-api-docs/assets/widgets/animated_default_text_style.mp4}
 class AnimatedDefaultTextStyle extends ImplicitlyAnimatedWidget {
   /// Creates a widget that animates the default text style implicitly.
   ///
@@ -1186,6 +1231,10 @@ class _AnimatedDefaultTextStyleState extends AnimatedWidgetBaseState<AnimatedDef
 /// because it is being driven by an [AnimatedTheme]).
 ///
 /// The [shape] is not animated.
+///
+/// Here's an illustration of what using this widget looks like, using a [curve]
+/// of [Curves.fastOutSlowIn].
+/// {@animation 250 266 https://flutter.github.io/assets-for-api-docs/assets/widgets/animated_physical_model.mp4}
 class AnimatedPhysicalModel extends ImplicitlyAnimatedWidget {
   /// Creates a widget that animates the properties of a [PhysicalModel].
   ///