Moving API doc asset URLs to point to the new location. (#17697)

I'm moving the assets in the assets-for-api-docs repo to a slightly different location to help with organization in that repo, so this PR points the doc URLs to the new location. The old assets won't be removed until this PR makes its way to the API docs website.

No documentation or code changes here, other than changing doc image URLs.

dev/manual_tests/lib/color_testing_demo.dart

@@ -22,9 +22,9 @@ class ColorDemoHome extends StatelessWidget {
       body: new ListView(
         padding: const EdgeInsets.all(5.0),
         children: <Widget>[
-          new Image.network('https://flutter.github.io/assets-for-api-docs/tests/colors/gbr.png'),
-          new Image.network('https://flutter.github.io/assets-for-api-docs/tests/colors/tf.png'),
-          new Image.network('https://flutter.github.io/assets-for-api-docs/tests/colors/wide-gamut.png'),
+          new Image.network('https://flutter.github.io/assets-for-api-docs/assets/tests/colors/gbr.png'),
+          new Image.network('https://flutter.github.io/assets-for-api-docs/assets/tests/colors/tf.png'),
+          new Image.network('https://flutter.github.io/assets-for-api-docs/assets/tests/colors/wide-gamut.png'),
           const GradientRow(leftColor: const Color(0xFFFF0000), rightColor: const Color(0xFF00FF00)),
           const GradientRow(leftColor: const Color(0xFF0000FF), rightColor: const Color(0xFFFFFF00)),
           const GradientRow(leftColor: const Color(0xFFFF0000), rightColor: const Color(0xFF0000FF)),

examples/flutter_gallery/lib/demo/video_demo.dart

@@ -11,7 +11,7 @@ import 'package:device_info/device_info.dart';
 
 // TODO(sigurdm): This should not be stored here.
 const String beeUri =
-    'https://flutter.github.io/assets-for-api-docs/videos/bee.mp4';
+    'https://flutter.github.io/assets-for-api-docs/assets/videos/bee.mp4';
 
 class VideoCard extends StatelessWidget {
   final VideoPlayerController controller;

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

@@ -79,7 +79,7 @@ class _ToolbarContainerLayout extends SingleChildLayoutDelegate {
 ///
 /// ![The leading widget is in the top left, the actions are in the top right,
 /// the title is between them. The bottom is, naturally, at the bottom, and the
-/// flexibleSpace is behind all of them.](https://flutter.github.io/assets-for-api-docs/material/app_bar.png)
+/// flexibleSpace is behind all of them.](https://flutter.github.io/assets-for-api-docs/assets/material/app_bar.png)
 ///
 /// If the [leading] widget is omitted, but the [AppBar] is in a [Scaffold] with
 /// a [Drawer], then a button will be inserted to open the drawer. Otherwise, if

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

@@ -49,7 +49,7 @@ import 'theme.dart';
 ///
 /// ![A card with a slight shadow, consisting of two rows, one with an icon and
 /// some text describing a musical, and the other with buttons for buying
-/// tickets or listening to the show.](https://flutter.github.io/assets-for-api-docs/material/card.png)
+/// tickets or listening to the show.](https://flutter.github.io/assets-for-api-docs/assets/material/card.png)
 ///
 /// See also:
 ///

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

@@ -129,11 +129,11 @@ abstract class InteractiveInkFeatureFactory {
 /// The first diagram shows how it looks if the [InkResponse] is relatively
 /// large:
 ///
-/// ![The highlight is a disc centered in the box, smaller than the child widget.](https://flutter.github.io/assets-for-api-docs/material/ink_response_large.png)
+/// ![The highlight is a disc centered in the box, smaller than the child widget.](https://flutter.github.io/assets-for-api-docs/assets/material/ink_response_large.png)
 ///
 /// The second diagram shows how it looks if the [InkResponse] is small:
 ///
-/// ![The highlight is a disc overflowing the box, centered on the child.](https://flutter.github.io/assets-for-api-docs/material/ink_response_small.png)
+/// ![The highlight is a disc overflowing the box, centered on the child.](https://flutter.github.io/assets-for-api-docs/assets/material/ink_response_small.png)
 ///
 /// The main thing to notice from these diagrams is that the splashes happily
 /// exceed the bounds of the widget (because [containedInkWell] is false).
@@ -142,7 +142,7 @@ abstract class InteractiveInkFeatureFactory {
 /// [highlightShape] of [BoxShape.rectangle] with [containedInkWell] set to
 /// true. These are the values used by [InkWell].
 ///
-/// ![The highlight is a rectangle the size of the box.](https://flutter.github.io/assets-for-api-docs/material/ink_well.png)
+/// ![The highlight is a rectangle the size of the box.](https://flutter.github.io/assets-for-api-docs/assets/material/ink_well.png)
 ///
 /// The [InkResponse] widget must have a [Material] widget as an ancestor. The
 /// [Material] widget is where the ink reactions are actually painted. This
@@ -565,7 +565,7 @@ class _InkResponseState<T extends InkResponse> extends State<T> with AutomaticKe
 /// The following diagram shows how an [InkWell] looks when tapped, when using
 /// default values.
 ///
-/// ![The highlight is a rectangle the size of the box.](https://flutter.github.io/assets-for-api-docs/material/ink_well.png)
+/// ![The highlight is a rectangle the size of the box.](https://flutter.github.io/assets-for-api-docs/assets/material/ink_well.png)
 ///
 /// The [InkWell] widget must have a [Material] widget as an ancestor. The
 /// [Material] widget is where the ink reactions are actually painted. This

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

@@ -15,30 +15,30 @@ import 'basic_types.dart';
 enum BoxFit {
   /// Fill the target box by distorting the source's aspect ratio.
   ///
-  /// ![](https://flutter.github.io/assets-for-api-docs/painting/box_fit_fill.png)
+  /// ![](https://flutter.github.io/assets-for-api-docs/assets/painting/box_fit_fill.png)
   fill,
 
   /// As large as possible while still containing the source entirely within the
   /// target box.
   ///
-  /// ![](https://flutter.github.io/assets-for-api-docs/painting/box_fit_contain.png)
+  /// ![](https://flutter.github.io/assets-for-api-docs/assets/painting/box_fit_contain.png)
   contain,
 
   /// As small as possible while still covering the entire target box.
   ///
-  /// ![](https://flutter.github.io/assets-for-api-docs/painting/box_fit_cover.png)
+  /// ![](https://flutter.github.io/assets-for-api-docs/assets/painting/box_fit_cover.png)
   cover,
 
   /// Make sure the full width of the source is shown, regardless of
   /// whether this means the source overflows the target box vertically.
   ///
-  /// ![](https://flutter.github.io/assets-for-api-docs/painting/box_fit_fitWidth.png)
+  /// ![](https://flutter.github.io/assets-for-api-docs/assets/painting/box_fit_fitWidth.png)
   fitWidth,
 
   /// Make sure the full height of the source is shown, regardless of
   /// whether this means the source overflows the target box horizontally.
   ///
-  /// ![](https://flutter.github.io/assets-for-api-docs/painting/box_fit_fitHeight.png)
+  /// ![](https://flutter.github.io/assets-for-api-docs/assets/painting/box_fit_fitHeight.png)
   fitHeight,
 
   /// Align the source within the target box (by default, centering) and discard
@@ -46,7 +46,7 @@ enum BoxFit {
   ///
   /// The source image is not resized.
   ///
-  /// ![](https://flutter.github.io/assets-for-api-docs/painting/box_fit_none.png)
+  /// ![](https://flutter.github.io/assets-for-api-docs/assets/painting/box_fit_none.png)
   none,
 
   /// Align the source within the target box (by default, centering) and, if
@@ -56,7 +56,7 @@ enum BoxFit {
   /// This is the same as `contain` if that would shrink the image, otherwise it
   /// is the same as `none`.
   ///
-  /// ![](https://flutter.github.io/assets-for-api-docs/painting/box_fit_scaleDown.png)
+  /// ![](https://flutter.github.io/assets-for-api-docs/assets/painting/box_fit_scaleDown.png)
   scaleDown,
 }
 

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

@@ -311,9 +311,9 @@ class LinearGradient extends Gradient {
   ///
   /// For details, see [TileMode].
   ///
-  /// ![](https://flutter.github.io/assets-for-api-docs/dart-ui/tile_mode_clamp_linear.png)
-  /// ![](https://flutter.github.io/assets-for-api-docs/dart-ui/tile_mode_mirror_linear.png)
-  /// ![](https://flutter.github.io/assets-for-api-docs/dart-ui/tile_mode_repeated_linear.png)
+  /// ![](https://flutter.github.io/assets-for-api-docs/assets/dart-ui/tile_mode_clamp_linear.png)
+  /// ![](https://flutter.github.io/assets-for-api-docs/assets/dart-ui/tile_mode_mirror_linear.png)
+  /// ![](https://flutter.github.io/assets-for-api-docs/assets/dart-ui/tile_mode_repeated_linear.png)
   final TileMode tileMode;
 
   @override
@@ -532,9 +532,9 @@ class RadialGradient extends Gradient {
   ///
   /// For details, see [TileMode].
   ///
-  /// ![](https://flutter.github.io/assets-for-api-docs/dart-ui/tile_mode_clamp_radial.png)
-  /// ![](https://flutter.github.io/assets-for-api-docs/dart-ui/tile_mode_mirror_radial.png)
-  /// ![](https://flutter.github.io/assets-for-api-docs/dart-ui/tile_mode_repeated_radial.png)
+  /// ![](https://flutter.github.io/assets-for-api-docs/assets/dart-ui/tile_mode_clamp_radial.png)
+  /// ![](https://flutter.github.io/assets-for-api-docs/assets/dart-ui/tile_mode_mirror_radial.png)
+  /// ![](https://flutter.github.io/assets-for-api-docs/assets/dart-ui/tile_mode_repeated_radial.png)
   final TileMode tileMode;
 
   @override