Specify what image formats are supported in the API docs. (#13717)

Depends on https://github.com/flutter/engine/pull/4483

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

@@ -43,7 +43,7 @@ export 'dart:ui' show
   hashList;
 
 // Intentionally not exported:
-//  - Image, decodeImageFromDataPipe, decodeImageFromList:
+//  - Image, instantiateImageCodec, decodeImageFromList:
 //      We use ui.* to make it very explicit that these are low-level image APIs.
 //      Generally, higher layers provide more reasonable APIs around images.
 //  - lerpDouble:

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

@@ -168,6 +168,8 @@ class ImageConfiguration {
 /// The type argument does not have to be specified when using the type as an
 /// argument (where any image provider is acceptable).
 ///
+/// The following image formats are supported: {@macro flutter.dart:ui.imageFormats}
+///
 /// ## Sample code
 ///
 /// The following shows the code required to write a widget that fully conforms

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

@@ -92,6 +92,8 @@ Future<Null> precacheImage(ImageProvider provider, BuildContext context, { Size
 ///  * [new Image.file], for obtaining an image from a [File].
 ///  * [new Image.memory], for obtaining an image from a [Uint8List].
 ///
+/// The following image formats are supported: {@macro flutter.dart:ui.imageFormats}
+///
 /// To automatically perform pixel-density-aware asset resolution, specify the
 /// image using an [AssetImage] and make sure that a [MaterialApp], [WidgetsApp],
 /// or [MediaQuery] widget exists above the [Image] widget in the widget tree.