Linking Higher & Lower Class Docs (#29758)

* Adding linking between higher and lower level classes in the API Docs. ref:#22859

* Fixed links between classes.

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

@@ -64,8 +64,8 @@ _ColorsAndStops _interpolateColorsAndStops(
 ///
 /// See also:
 ///
-///  * [dart:ui.Gradient], the class in the [dart:ui] library that is
-///    encapsulated by this class and its subclasses.
+///  * [Gradient](https://api.flutter.dev/flutter/dart-ui/Gradient-class.html), the class in the [dart:ui] library.
+///
 @immutable
 abstract class Gradient {
   /// Initialize the gradient's colors and stops.

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

@@ -44,6 +44,10 @@ import 'text_style.dart';
 /// leading plus ascent. Each line's spacing below the baseline will be at least as
 /// tall as the half leading plus descent.
 ///
+/// See also:
+///
+///  * [StrutStyle](https://api.flutter.dev/flutter/dart-ui/StrutStyle-class.html), the class in the [dart:ui] library.
+///
 /// ### Fields and their default values.
 
 // ///////////////////////////////////////////////////////////////////////////

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

@@ -292,6 +292,8 @@ const String _kColorBackgroundWarning = 'Cannot provide both a backgroundColor a
 ///  * [RichText], the widget for showing a paragraph of mix-style text.
 ///  * [TextSpan], the class that wraps a [TextStyle] for the purposes of
 ///    passing it to a [RichText].
+///  * [TextStyle](https://api.flutter.dev/flutter/dart-ui/TextStyle-class.html), the class in the [dart:ui] library.
+///
 @immutable
 class TextStyle extends Diagnosticable {
   /// Creates a text style.

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

@@ -132,6 +132,8 @@ Future<void> precacheImage(
 ///  * [new Ink.image], which is the preferred way to show an image in a
 ///    material application (especially if the image is in a [Material] and will
 ///    have an [InkWell] on top of it).
+///  * [Image](https://api.flutter.dev/flutter/dart-ui/Image-class.html), the class in the [dart:ui] library.
+///
 class Image extends StatefulWidget {
   /// Creates a widget that displays an image.
   ///